[libFuzzer] exit without running atexit handlers in libfuzzer's crash handler

Summary:
It's not safe to assume that atexit handlers can be run once the app crashed.

Patch by Jochen Eisinger.

Reviewers: kcc, vitalybuka

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32640

llvm-svn: 302076