[libFuzzer] Make MutateWithMask work when the Mask is shorter than the input.

Summary:
Before this change, MutateWithMask used to assert that Mask should be
of sufficient length (>= Size of the input). However, in real cases we may have
inputs that are longer than the Mask they have inherited from the based inputs.

Reviewers: kcc, morehouse

Reviewed By: kcc

Subscribers: delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D60571

llvm-svn: 358207