[Bitcode] Check minimum size of constant GEP record

Checking this early, because we may end up reading up to two
records before the operands.

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2676,6 +2676,8 @@ Error BitcodeReader::parseConstants() {
     case bitc::CST_CODE_CE_GEP: // [ty, n x operands]
     case bitc::CST_CODE_CE_GEP_WITH_INRANGE_INDEX: { // [ty, flags, n x
                                                      // operands]
+      if (Record.size() < 2)
+        return error("Constant GEP record must have at least two elements");
       unsigned OpNum = 0;
       Type *PointeeType = nullptr;
       if (BitCode == bitc::CST_CODE_CE_GEP_WITH_INRANGE_INDEX ||