MIPS/GAS: Fix null pointer dereferences in R6 PC-relative relocation checks

MIPS/GAS: Fix null pointer dereferences in R6 PC-relative relocation checks

Avoid segmentation faults in alignment checks made in `md_apply_fix' for
BFD_RELOC_MIPS_18_PCREL_S3 and BFD_RELOC_MIPS_19_PCREL_S2 relocations
caused by dereferencing `fixP->fx_addsy' which will be null if the
relocation processed has been fully resolved.

	gas/
	* config/tc-mips.c (md_apply_fix) <BFD_RELOC_MIPS_18_PCREL_S3>
	<BFD_RELOC_MIPS_19_PCREL_S2>: Avoid null pointer dereferences
	via `fixP->fx_addsy'.