Error out of ParseBitcodeInto(Module*) if we haven't read a Module

Summary:
Without this check the following case failed:

Skip a SubBlock which is not a MODULE_BLOCK_ID nor a BLOCKINFO_BLOCK_ID
Got to end of file

TheModule would still be == nullptr, and we would subsequentially fail
when materializing the Module (assert at the start of
BitcodeReader::MaterializeModule).

Bug found with AFL.

Reviewers: dexonsmith, rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9014

llvm-svn: 234887

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -3063,8 +3063,12 @@ std::error_code BitcodeReader::ParseBitcodeInto(Module *M,
   // We expect a number of well-defined blocks, though we don't necessarily
   // need to understand them all.
   while (1) {
-    if (Stream.AtEndOfStream())
-      return std::error_code();
+    if (Stream.AtEndOfStream()) {
+      if (TheModule)
+        return std::error_code();
+      // We didn't really read a proper Module.
+      return Error("Malformed IR file");
+    }
 
     BitstreamEntry Entry =
       Stream.advance(BitstreamCursor::AF_DontAutoprocessAbbrevs);