Fail when string merge can't alloc memory

Fail when string merge can't alloc memory

I was looking at Debian bug #874674 again today, and think I might
have spotted the problem.  It appears that merge.c tries to cope with
memory allocation failures in some circumstances, but doesn't quite
manage to get everything right.  This patch will make ld report memory
allocation failures instead of silently not merging strings.

	* merge.c (merge_strings): Return FALSE on malloc failure.
	(_bfd_merge_sections): Return failures from record_section and
	merge_strings.