ppc: initialize needComma to false. issue reported by Coverity
mips: verify if RegDecoder can get NULL value. issue reported by Coverity
fix a negative array index read in PPC_alias_insn(). issue reported by Coverity
mips: fix out-of-bounds read error in Mips_reg_name(). issue reported by Coverity
x86: fix Out-of-bounds read error in is16BitEquivalent(). issue reported by Coverity