upnp: refactor using C++ try/catch

The try/catch for exception safety was already needed there, and this
simplify the code a lot, so this moves the allocation into unique_ptr to
benefit from RAII destructors.

In addition, this commit change the call to free(p_sys) into the correct
C++ delete, avoiding a mismatch new/free.

Introduced in 0d89fe3f.