LLVMFuzzerTestOneInput is not supposed to ever return something other
than 0. Instead, the whole process should abort in case of a bug being
detected.

Hence, this replaces the previous returning of 1 (which triggered a
libfuzzer assertion downstream, that was detected by clusterfuzz as the
root cause of the error) by an abort (which should let clusterfuzz
detect the actual cause of the error in the panic message)