[libfuzzer] Compare TotalNumberOfRuns with MaxNumberOfRuns when testing a memory leak.

Summary:
Fuzzer::TryDetectingAMemoryLeak may call ExecuteCallback which would
increment TotalNumberOfRuns, but it doesn't respect Options.MaxNumberOfRuns
value specified by a user.

Context: https://github.com/google/oss-fuzz/issues/822#issuecomment-328153970

Reviewers: kcc

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D37632

llvm-svn: 312993