XCOFF linker segmentation fault

XCOFF linker segmentation fault

The XCOFF linker temporarily trims the output bfd section list,
without adjusting section_count to suit.  This is a little rude, but
the dwarf line number code can easily cope with this situation.  So
check for a NULL end of list as well as limiting the saved section
VMAs to the first section_count list entries.

Also fixes
-FAIL: Weak test 3 (main, static) (32-bit)
-FAIL: Weak test 3 (main, static) (64-bit)

	PR 24596
	* dwarf2.c (save_section_vma, section_vma_same): Check for NULL
	end of section list as well as section_count.
	* xcofflink.c (xcoff_link_add_symbols): Fix temporarily changed
	section list before returning error.