fix crash when P.Arch() was used but the cache got remapped

diff --cc apt-pkg/pkgcachegen.cc
Simple merge

diff --cc debian/changelog
index 05502a9,e17b1ab..2d3f1a9
--- 1/debian/changelog
--- 2/debian/changelog
+++ b/debian/changelog
@@@ -1,36 -1,5 +1,37 @@@
 -apt (0.8.15.6) unstable; urgency=low
 +apt (0.8.16~exp5) UNRELEASED; urgency=low
  
-  -- Michael Vogt <michael.vogt@ubuntu.com>  Fri, 29 Jul 2011 13:44:01 +0200
 +  * apt-pkg/makefile:
 +    - install sha256.h compat header
 +  * apt-pkg/pkgcachegen.{cc,h}:
 +    - use ref-to-ptr semantic in NewDepends() to ensure that the   
 +      libapt does not segfault if the cache is remapped in between
 +      (LP: #812862)
++    - fix crash when P.Arch() was used but the cache got remapped
 +
++ -- Michael Vogt <mvo@debian.org>  Mon, 01 Aug 2011 15:31:55 +0200
 +
 +apt (0.8.16~exp4) experimental; urgency=low
 +
 +  [ Julian Andres Klode ]
 +  * apt-pkg/pkgcache.h:
 +    - [ABI break] Add pkgCache::Header::CacheFileSize, storing the cache size
 +  * apt-pkg/pkgcachegen.cc:
 +    - Write the file size to the cache
 +  * apt-pkg/pkgcache.cc:
 +    - Check that cache is at least CacheFileSize bytes large (LP: #16467)
 +  
 +  [ Michael Vogt ]
 +  * merged latest fixes from debian-sid
 +  * apt-pkg/cdrom.{cc,h}:
 +    - cleanup old ABI break avoidance hacks
 +  * [ABI break] apt-pkg/acquire-item.{cc,h}:
 +    - cleanup around OptionalIndexTarget and SubIndexTarget
 +  * [ABI break] merged patch from Jonathan Thomas to have a new
 +    RecordField() function in the pkgRecorder parser. Many thanks
 +    Thomas
 +  * [ABI break] merge patch from Jonathan Thomas to speed up the
 +    depcache by caching the install-recommends and install-suggests
 +    values
    * apt-pkg/contrib/fileutl.{cc,h}:
      - add GetModificationTime() helper
    * apt-pkg/pkgcachegen.cc:
@@@ -38,74 -7,14 +39,74 @@@
        that changes in the ordering there will be honored by apt
    * apt-pkg/sourcelist.{cc,h}:
      - add pkgSourceList::GetLastModifiedTime() helper
 -  * apt-pkg/pkgcachegen.{cc,h}:
 -    - use ref-to-ptr semantic in NewDepends() to ensure that the   
 -      libapt does not segfault if the cache is remapped in between
 -      (LP: #812862)
  
 - -- Michael Vogt <mvo@debian.org>  Tue, 12 Jul 2011 11:54:47 +0200
 + -- Michael Vogt <mvo@debian.org>  Thu, 28 Jul 2011 16:57:08 +0200
 +
 +apt (0.8.16~exp3) experimental; urgency=low
 +
 +  [ David Kalnischkies ]
 +  * apt-pkg/pkgcache.h:
 +    - readd All{Foreign,Allowed} as suggested by Julian to
 +      remain strictly API compatible
 +  * apt-pkg/acquire*.{cc,h}:
 +    - try even harder to support really big files in the fetcher by
 +      converting (hopefully) everything to 'long long' (Closes: #632271)
 +  * ftparchive/writer.cc:
 +    - generate all checksums in one run over the file for Release
 +  * cmdline/apt-get.cc:
 +    - add an --assume-no option for testing to say 'no' to everything
 +  * apt-pkg/deb/debmetaindex.cc:
 +    - add trusted=yes option to mark unsigned (local) repository as trusted
 +      based on a patch from Ansgar Burchardt, thanks a lot! (Closes: #596498)
 +
 +  [ Michael Vogt ]
 +  * merge fixes from the debian/unstable upload
 +  * merge lp:~mvo/apt/sha512-template to get fixes for the 
 +    sha1/md5 verifiation (closes: #632520)
 +
 + -- Michael Vogt <mvo@debian.org>  Fri, 15 Jul 2011 09:56:17 +0200
 +
 +apt (0.8.16~exp2) experimental; urgency=low
 +
 +  [ David Kalnischkies ]
 +  * [ABI-Break] Implement EDSP in libapt-pkg so that all front-ends which
 +    use the internal resolver can now be used also with external
 +    ones as the usage is hidden in between the old API
 +  * provide two edsp solvers in apt-utils:
 +    - 'dump' to quickly output a complete scenario and
 +    - 'apt' to use the internal as an external resolver
 +  * apt-pkg/pkgcache.h:
 +    - clean up mess with the "all" handling in MultiArch to
 +      fix LP: #733741 cleanly for everyone now
 +  * apt-pkg/depcache.cc:
 +    - use a boolean instead of an int for Add/Remove in AddStates
 +      similar to how it works with AddSizes
 +    - let the Mark methods return if their marking was successful
 +    - if a Breaks can't be upgraded, remove it. If it or a Conflict
 +      can't be removed the installation of the breaker fails.
 +  * cmdline/apt-get.cc:
 +    - do not discard the error messages from the resolver and instead
 +      only show the general 'Broken packages' message if nothing else
 +
 +  [ Stefano Zacchiroli ]
 +  * doc/external-dependency-solver-protocol.txt:
 +    - describe EDSP and the configuration interface around it
 +  
 +  [ Michael Vogt ]
 +  * [ABI-Break] merge lp:~mvo/apt/sha512-template to add support for sha512
 +  * [ABI-Break] merge lp:~mvo/apt/dpointer to support easier extending
 +    without breaking the ABI
 +  * increase ABI version and update package names
 +
 + -- Michael Vogt <mvo@debian.org>  Wed, 29 Jun 2011 13:57:28 +0200
 +
 +apt (0.8.16~exp1) experimental; urgency=low
 +
 +  * merged with the debian/unstable upload
 +
 + -- Michael Vogt <mvo@debian.org>  Wed, 29 Jun 2011 12:40:31 +0200
  
- apt (0.8.15.5) UNRELEASED; urgency=low
+ apt (0.8.15.5) unstable; urgency=low
  
    [ David Kalnischkies ]
    * apt-pkg/deb/deblistparser.cc: