MIPS/BFD: Suppress attribute checks for null input

MIPS/BFD: Suppress attribute checks for null input

We currently special-case the handling of attribute checks on input
objects and make them even before we check a given input object actually
contains any sections.  This does not add value as empty objects do not
cause a compatibility concern and we already make this observation for
other properties such as ELF file header flags.  Moreover the attributes
themselves are stored in a `.gnu.attributes' section so the absence of
any section (except from a few special cases) implies there have been no
attributes provided either.  Therefore it is safe to move the attribute
checks later on, after the null-section check has been made.

	bfd/
	* elfxx-mips.c (_bfd_mips_elf_merge_private_bfd_data): Suppress
	attribute checks for null input.