[fuzzer] Fix -Wunreachable-code-aggressive warning in FuzzerDriver.cpp

compiler-rt/lib/fuzzer/FuzzerDriver.cpp

if (U.size() < 2) {
     Printf("INFO: The input is small enough, exiting\n");
     exit(0);
   }
   F->SetMaxInputLen(U.size());
   F->SetMaxMutationLen(U.size() - 1);
   F->MinimizeCrashLoop(U);
   Printf("INFO: Done MinimizeCrashInputInternalStep, no crashes found\n");
   exit(0);
-  return 0;
 }
 
 void Merge(Fuzzer *F, FuzzingOptions &Options,
            const std::vector<std::string> &Args,
            const std::vector<std::string> &Corpora, const char *CFPathOrNull) {
   if (Corpora.size() < 2) {
     Printf("INFO: Merge requires two or more corpus dirs\n");
     exit(0);