fix compilation of glil to assembly

fix compilation of glil to assembly

* libguile/vm-i-scheme.c (VM_VALIDATE_STRUCT): Fix the error message if
  the value was not a struct.

* module/system/base/compile.scm (find-language-joint): Default to
  joining at the target language.
  (default-language-joiner): Allow sequences of one compiled expression
  to pass through.  Otherwise error as before.
  (read-and-parse): New helper; actually parses.
  (read-and-compile): Use read-and-parse, and fall back to
  default-language-joiner.

Thanks to Nala Ginrut for the report.