fix multiple validation issues in dns response label parsing

Due to an error introduced in commit fcc522c92335783293ac19df318415cd97fbf66b,
checking of the remaining output buffer space was not performed correctly,
allowing malformed input to write past the end of the buffer.

In addition, the loop detection logic failed to account for the possibility
of infinite loops with no output, which would hang the function.

The output size is now limited more strictly so only names with valid length
are accepted.