[Frontend] Return an error on bad inputs to PrecompiledPreabmle

Summary:
Instead of failing with assertions. Fixes a crash found by oss-fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=12865

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62137

llvm-svn: 361376