Initial implementation of a record-level bitcode fuzzer.

include/llvm/Bitcode/NaCl/NaClBitstreamWriter.h

Index: include/llvm/Bitcode/NaCl/NaClBitstreamWriter.h
diff --git a/include/llvm/Bitcode/NaCl/NaClBitstreamWriter.h b/include/llvm/Bitcode/NaCl/NaClBitstreamWriter.h
index 39a5009c865ba196a123d3947dfa8c1abf4cf2e0..331c3d181cfaf0886ddbb8ab014a23a4e7350d02 100644
--- a/include/llvm/Bitcode/NaCl/NaClBitstreamWriter.h
+++ b/include/llvm/Bitcode/NaCl/NaClBitstreamWriter.h
@@ -431,9 +431,9 @@ public:
   /// Returns a pointer to the abbreviation currently associated with
   /// the abbreviation index. Returns nullptr if no such abbreviation.
   const NaClBitCodeAbbrev *getAbbreviation(unsigned Index) const {
-    if (Index < naclbitc::FIRST_APPLICATION_ABBREV)
-      return nullptr;
-    if (Index >= BlockScope.back().AbbreviationIndexLimit)
+    if (Index < naclbitc::FIRST_APPLICATION_ABBREV
+        || BlockScope.empty()
+        || Index >= BlockScope.back().AbbreviationIndexLimit)
       return nullptr;
     unsigned AbbrevNo = Index - naclbitc::FIRST_APPLICATION_ABBREV;
     if (AbbrevNo >= CurAbbrevs.size())
@@ -494,6 +494,7 @@ public:
   /// method takes ownership of the specified abbrev.
   unsigned EmitAbbrev(NaClBitCodeAbbrev *Abbv) {
     assert(Abbv->isValid() && "Can't emit invalid abbreviation!");
+    assert(!BlockScope.empty() && "Can't emit abbreviations outside of blocks");
     // Emit the abbreviation as a record.
     EncodeAbbrev(Abbv);
     CurAbbrevs.push_back(Abbv);