When the function encounters a key other than "type" and then fails to skip the value (json_skip_value returns false), it returns NULL without freeing the key that was just allocated by json_string. This leaks the key string.
If json_string succeeds for the key but the following character is not ':' (the if (*p != ':') branch), the function returns NULL after freeing key. However, if json_string fails (returns false), the function returns NULL without freeing key because key may have been partially allocated or left as NULL. The json_string implementation is not shown, but if it allocates key before failing, this is a leak. Even if it doesn't, the pattern is fragile.