* libguile/foreign.c (sym_null_pointer_error): New variable.
(null_pointer_error): New function.
(scm_foreign_set_x): Raise an error if attempting to modify
NULL_POINTER.
(scm_foreign_to_bytevector): Use `null_pointer_error ()' instead of
`scm_misc_error ()'.
* test-suite/tests/foreign.test: New file.
* test-suite/Makefile.am (SCM_TESTS): Add tests/foreign.test.
* test-suite/lib.scm (exception:null-pointer-error): New variable.
