USE_MMAP fuzzed object file attacks

USE_MMAP fuzzed object file attacks

If mmap is used without sanity checking, then we'll get a SIGBUS if
an access is done to the mmap'd memory corresponding to a page past
end of file.

	* aoutx.h (aout_get_external_symbols): Check that mmap regions
	are within file contents.  Catch stringsize overflow.
	(some_aout_object_p): Don't clear already zeroed fields.  Tidy.
	* pdp11.c: As for aoutx.h.  Copy some fixes too.