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