Libfuzzer fix for Ctrl + c not working with -fork and -ignore_crashes=1

In some cases running Libfuzzer in fork mode with -ignore_crashes=1 counts ctrl+c as crash and restarts.

Thread: https://github.com/google/oss-fuzz/issues/4547

Credit: Marcel Boehme <marcel.boehme@acm.org>

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D130990