output commands: move !argc checks after argc gets decremented

Found through static analysis.

sway/commands/output/mode.c

@@ -36,11 +36,11 @@ struct cmd_results *output_cmd_mode(int argc, char **argv) {
 		}
 	} else {
 		// Format is 1234 4321
+		argc--; argv++;
 		if (!argc) {
 			return cmd_results_new(CMD_INVALID, "output",
 				"Missing mode argument (height).");
 		}
-		argc--; argv++;
 		output->height = strtol(*argv, &end, 10);
 		if (*end) {
 			return cmd_results_new(CMD_INVALID, "output",

sway/commands/output/position.c

@@ -27,11 +27,11 @@ struct cmd_results *output_cmd_position(int argc, char **argv) {
 		}
 	} else {
 		// Format is 1234 4321 (legacy)
+		argc--; argv++;
 		if (!argc) {
 			return cmd_results_new(CMD_INVALID, "output",
 				"Missing position argument (y).");
 		}
-		argc--; argv++;
 		config->handler_context.output_config->y = strtol(*argv, &end, 10);
 		if (*end) {
 			return cmd_results_new(CMD_INVALID, "output",