z8 and z80 coff_reloc16_extra_cases sanity checks

z8 and z80 coff_reloc16_extra_cases sanity checks

	* reloc16.c (bfd_coff_reloc16_get_relocated_section_contents):
	Use size_t variables.  Sanity check reloc address.  Handle
	errors from bfd_coff_reloc16_extra_cases.
	* coffcode.h (_bfd_coff_reloc16_extra_cases): Return bool, take
	size_t* args.
	(dummy_reloc16_extra_cases): Adjust to suit.  Don't abort.
	* coff-z80.c (extra_case): Sanity check reloc address.  Return
	errors.  Tidy formatting.  Use bfd_signed_vma temp var to
	check for reloc overflow.  Don't abort on unexpected reloc type,
	instead print an error and return false.
	* coff-z8k.c (extra_case): Likewise.
	* libcoff.h: Regenerate.