Release bfd_alloc memory in bfd_check_format_matches

Release bfd_alloc memory in bfd_check_format_matches

It's a little tricky.  We can release any memory back when we have a
match failure, but after a match success which we might want to
preserve for later use the high water mark must change to that of the
matched bfd.

	* format.c (bfd_check_format_matches): Add preserve_match.
	Save initial bfd state in "preserve", matched bfd state in
	"preserve_match".  Save just the first match.  Release
	bfd_alloc memory.  Restore and finish preserved state as
	appropriate on all function exit paths.