[libFuzzer]Add a test for defeating a hash sum.

Summary:
Add a test for a data followed by 4-byte hash value.
I use a slightly modified Jenkins hash function,
as described in https://en.wikipedia.org/wiki/Jenkins_hash_function

The modification is to ensure that hash(zeros) != 0.

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12648

llvm-svn: 247076