asan: mips_gprel_reloc segfault

Not just the undefined section has a NULL owner, the absolute section
has too.  Which means we can't find output_bfd for __gp.  Also, may as
well test directly for output_bfd == NULL.

	* coff-mips.c (mips_gprel_reloc): Don't segfault on any of
	bfd_is_const_section sections.