cmd: stop "mtu" from acting on a value it failed to parse

The handler threw away atoi_short()'s return and leant on the range test
alone. The range test cannot tell a failed parse from a small number, so
"mtu 1 99999" stopped at the partial 9999 and went through as a number
nobody typed. With the parse result checked, a failure is rejected with
the same message as an out-of-range value.

cmd_parser.c

@@ -736,8 +736,7 @@ void parse_mtu(void)
 		print_string("mtu [port] [size]\n");
 		return;
 	}
-	atoi_short(&mtu, cmd_words_b[2]);
-	if (mtu < 64 || mtu > 0x3fff) {
+	if (atoi_short(&mtu, cmd_words_b[2]) || mtu < 64 || mtu > 0x3fff) {
 		print_string("MTU must be 64..16383\n");
 		return;
 	}