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

[jochen]

Summary

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

Patch by Jochen Eisinger.

[jochen]

forgot to include the error exit code..

[jochen]

+glider

[vitalybuka]

LGTM

[vitalybuka]

Update description.

lib/Fuzzer/FuzzerLoop.cpp

void Fuzzer::CrashCallback() {
   if (EF->__sanitizer_print_stack_trace)
     EF->__sanitizer_print_stack_trace();
   Printf("NOTE: libFuzzer has rudimentary signal handlers.\n"
          "      Combine libFuzzer with AddressSanitizer or similar for better "
          "crash reports.\n");
   Printf("SUMMARY: libFuzzer: deadly signal\n");
   DumpCurrentUnit("crash-");
   PrintFinalStats();
-  exit(Options.ErrorExitCode);
+  _Exit(Options.ErrorExitCode);  // Stop right now.
 }
 
 void Fuzzer::InterruptCallback() {
   Printf("==%lu== libFuzzer: run interrupted; exiting\n", GetPid());
   PrintFinalStats();
   _Exit(0);  // Stop right now, don't perform any at-exit actions.
 }