asan: NULL dereference in _bfd_mips_final_write_processing

Fuzzed object files can easily have unexpected section names.  We
don't want to segfault on objcopy of any file accepted by the mips
object_p functions.  For objcopy, an assertion that "sec" is non-NULL
followed by deferencing "sec" is wrong.  So too is asserting that the
section name string starts with a particular prefix, and then blithely
accessing past the assumed prefix.

	* elfxx-mips.c (_bfd_mips_final_write_processing): Replace
	assertions with conditionals.  Don't bother testing for name
	non-NULL.