Fix missing port-table locking and bytevector output port segfault

Fix missing port-table locking and bytevector output port segfault

* libguile/r6rs-ports.c (make_bip, make_cbip, make_bop, make_cbop): Lock
  the port table.

* libguile/r6rs-ports.c (make_bop): Let the returned extraction
  procedure refer to the port's buffer instead of the port itself.  This
  fixes a segfault if the port is closed before the extraction procedure
  is called.
  (bop_proc_apply): Adapt accordingly.
* test-suite/tests/r6rs-ports.test (8.2.10 Output ports): Add testcase
  for extraction after close.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>