Make sure we don't resize(0) when we get a fwdref with Idx == UINT_MAX

Make it an error instead.

Bug found with AFL fuzz.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236190 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -790,6 +790,10 @@ Constant *BitcodeReaderValueList::getConstantFwdRef(unsigned Idx,
 }
 
 Value *BitcodeReaderValueList::getValueFwdRef(unsigned Idx, Type *Ty) {
+  // Bail out for a clearly invalid value. This would make us call resize(0)
+  if (Idx == UINT_MAX)
+    return nullptr;
+
   if (Idx >= size())
     resize(Idx + 1);