Keep samples resulting in crashes

[jvoisin]

The fuzzer should put samples that are resulting
in crashes or timeouts in the corpus, to be able to relaunch the fuzzer
once the issue is fixed, as well as to increase the corpus'
diversity in case it's be re-used by a different fuzzer.

[yevgenypats]

maybe this can be guarded via a flag as in libFuzzer this is not the default behaviour and I'm trying to adhere to that interface. Also, I had some use-case where I didn't want to input "crashes" (test-cases) that are not fixed in my corpus and know that my corpus shouldn't crash the fuzz target.

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