Verify sizes when trying to read a BitcodeAbbrevOp

Summary:
Make sure the abbrev operands are valid and that we can read/skip them
afterwards.

Bug found with AFL fuzz.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9030

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235595 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/BitstreamReader.h

@@ -198,6 +198,8 @@ class BitstreamCursor {
 
 
 public:
+  static const size_t MaxChunkSize = sizeof(word_t) * 8;
+
   BitstreamCursor() { init(nullptr); }
 
   explicit BitstreamCursor(BitstreamReader &R) { init(&R); }
@@ -335,7 +337,7 @@ public:
   }
 
   word_t Read(unsigned NumBits) {
-    static const unsigned BitsInWord = sizeof(word_t) * 8;
+    static const unsigned BitsInWord = MaxChunkSize;
 
     assert(NumBits && NumBits <= BitsInWord &&
            "Cannot return zero or more than BitsInWord bits!");