1. The result of vsnprintf was cast to size_t, which could lead to a signed-to-unsigned overflow error.
2. There was no handling for cases where vsnprintf returns a negative value or a value greater than NGX_MAX_ERROR_STR.

Looks like libxml does not generate the messages which could trigger these issue. Getting an error out of vsnprintf is also non-trivial. But anyway I agree we should handle all possible cases correctly.