[libFuzzer] Fallback to default Mutate when MutateWithMask fails.

Summary:
In case the current corpus input doesn't have bytes going into the
focus function, MutateWithMask is useless and may fail gently, allowing the
default mutation routine happen, rather than crashing on an assertion.

For more context and the initial fix suggestion, see:
https://github.com/google/oss-fuzz/issues/1632#issuecomment-481862879

Reviewers: kcc, morehouse

Reviewed By: kcc

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D60567

llvm-svn: 358190