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

[filcab]

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.

lib/Bitcode/Reader/BitcodeReader.cpp

if (Stream.Read(8) != 'B' ||
       Stream.Read(4) != 0xC ||
       Stream.Read(4) != 0xE ||
       Stream.Read(4) != 0xD)
     return Error("Invalid bitcode signature");
 
   // We expect a number of well-defined blocks, though we don't necessarily
   // need to understand them all.
   while (1) {
-    if (Stream.AtEndOfStream())
+    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);
 
     switch (Entry.Kind) {
     case BitstreamEntry::Error:
       return Error("Malformed block");
     case BitstreamEntry::EndBlock:

test/Bitcode/invalid.test

 
 
 EXTRACT-ARRAY: EXTRACTVAL: Invalid array index
 EXTRACT-STRUCT: EXTRACTVAL: Invalid struct index
 EXTRACT-IDXS: EXTRACTVAL: Invalid type
 INSERT-ARRAY: INSERTVAL: Invalid array index
 INSERT-STRUCT: INSERTVAL: Invalid struct index
 INSERT-IDXS: INSERTVAL: Invalid type
+
+RUN: not llvm-dis -disable-output %p/Inputs/invalid-no-proper-module.bc 2>&1 | \
+RUN:   FileCheck --check-prefix=NO-MODULE %s
+
+NO-MODULE: Malformed IR file