MIPS/BFD: Fix assertion in `mips_elf_sort_hash_table'

Move the assertion on non-NULL `htab' in `mips_elf_sort_hash_table' to
the beginning, before the pointer is dereferenced (`mips_elf_hash_table
(info)' and `elf_hash_table (info)' both point to the same memory
location, differently typed).

	bfd/
	* elfxx-mips.c (mips_elf_sort_hash_table): Move assertion on
	non-NULL `htab' to the beginning.