XCOFF archive sanity check

XCOFF archive elements are in a linked list.  Add a little more sanity
checking.  This of course doesn't stop the fuzzers finding a way to
make a loop, but this check is cheap.

	* coff-rs6000.c (_bfd_xcoff_openr_next_archived_file): Sanity
	check that next element isn't pointing back to the header.