[BlockExtractor] Avoid assert with wrong line format

Summary:
When the line format is wrong, we may end up accessing out of bound
memory. eg: the test with invalide line will cause assert.
Assertion `idx < size()' failed

The fix is to report fatal when we found mismatched line format.

Reviewers: qcolombet, volkan

Reviewed By: qcolombet

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66444

llvm-svn: 369389