Fix segfault in jack_connect

When jack_connect is called without any argument, it causes a segfault
in

                snprintf( portA, sizeof(portA), "%s", argv[argc-1] );
                snprintf( portB, sizeof(portB), "%s", argv[argc-2] );

Reported in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=662199

example-clients/connect.c

@@ -128,7 +128,10 @@ main (int argc, char *argv[])
 		return 1;
 	}
 
-	if (argc < 3) show_usage(my_name);
+	if (argc < 3) {
+		show_usage(my_name);
+		return 1;
+	}
 
 	/* try to become a client of the JACK server */