Don't segfault in mips reloc special_functions

Don't segfault in mips reloc special_functions

A symbol defined in a section from a shared library will have a NULL
section->output_section during linking.

	* elf32-mips.c (gprel32_with_gp): Don't segfault on NULL
	symbol->section->output_section.
	* elf64-mips.c (mips_elf64_gprel32_reloc): Likewise.
	* elfn32-mips.c (mips_elf_gprel16_reloc): Likewise.
	(mips_elf_literal_reloc, mips_elf_gprel32_reloc): Likewise.
	(gprel32_with_gp, mips16_gprel_reloc): Likewise.
	* elfxx-mips.c (_bfd_mips_elf_gprel16_with_gp): Likewise.
	(_bfd_mips_elf_generic_reloc): Likewise.