cmd/bar/colors: fix dereference of null pointer

`!*rgba` tests if the first byte of rgba isn't `'\0'`.
`hex_to_rgba_hex` returns NULL if `parse_color` fails. There's a null
pointer dereference in that case. The intended behavior is `!rgba`.

sway/commands/bar/colors.c

@@ -24,6 +24,9 @@ static char *hex_to_rgba_hex(const char *hex) {
 		return NULL;
 	}
 	char *rgba = malloc(10);
+	if (!rgba) {
+		return NULL;
+	}
 	snprintf(rgba, 10, "#%08x", color);
 	return rgba;
 }
@@ -36,7 +39,7 @@ static struct cmd_results *parse_single_color(char **color,
 	}
 
 	char *rgba = hex_to_rgba_hex(argv[0]);
-	if (!*rgba) {
+	if (!rgba) {
 		return cmd_results_new(CMD_INVALID, "Invalid color: %s", argv[0]);
 	}