cmd_background: fix leak on error

Found through static analysis.

sway/commands/output/background.c

@@ -83,6 +83,7 @@ struct cmd_results *output_cmd_background(int argc, char **argv) {
 			char *conf = strdup(config->current_config);
 			if (!conf) {
 				wlr_log(L_ERROR, "Failed to duplicate string");
+				free(src);
 				return cmd_results_new(CMD_FAILURE, "output",
 						"Unable to allocate resources");
 			}