Return after calling cmdarg_err().

cmdarg_err() prints a message, but it doesn't exit.

Change-Id: I887d96bce483f873a4375cb6b5254915d014f1b1
Reviewed-on: https://code.wireshark.org/review/17509
Reviewed-by: Guy Harris <guy@alum.mit.edu>

randpkt.c

@@ -172,6 +172,7 @@ main(int argc, char **argv)
 				produce_max_bytes = get_positive_int(optarg, "max bytes");
 				if (produce_max_bytes > 65536) {
 					cmdarg_err("max bytes is > 65536");
+					return 1;
 				}
 				break;