[libFuzzer] Report at most one crash per input.

Summary:
Fixes https://github.com/google/sanitizers/issues/788/, a deadlock
caused by multiple crashes happening at the same time.  Before printing
a crash report, we now test and set an atomic flag.  If the flag was
already set, the crash handler returns immediately.

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D46277

llvm-svn: 331310