[libFuzzer] Don't replace custom signal handlers.

Summary:
This allows to keep handlers installed by sanitizers.
In other cases third-party code can replace handlers after libFuzzer
initialization anyway.

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33522

git-svn-id: svn://svn.chromium.org/llvm-project/llvm/trunk/lib/Fuzzer@303828 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
2 files changed