Flush stdout before successful return from LLVMFuzzerTestOneInput().

Fixes a problem with diag lines from libFuzzer being interspersed
with test output.

lib/util/regress/fuzz/fuzz_sudo_conf.c

@@ -57,5 +57,7 @@ LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 
     unlink(tempfile);
 
+    fflush(stdout);
+
     return 0;
 }