[BitcodeReader] Validate OpNum, before accessing Record array.

Currently invalid bitcode files can cause a crash, when OpNum exceeds
the number of elements in Record, like in the attached bitcode file.

The test case was generated by clusterfuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15698

Reviewers: t.p.northover, thegameg, jfb

Reviewed By: jfb

Differential Revision: https://reviews.llvm.org/D64507

llvm-svn: 365750

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -4165,6 +4165,10 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
           popValue(Record, OpNum, NextValueNo, LHS->getType(), RHS))
         return error("Invalid record");
 
+      if (OpNum >= Record.size())
+        return error(
+            "Invalid record: operand number exceeded available operands");
+
       unsigned PredVal = Record[OpNum];
       bool IsFP = LHS->getType()->isFPOrFPVectorTy();
       FastMathFlags FMF;