upnp: fix null ptr access

A part of the patch [1] has been drop during a rebase.
This caused a access to a possible null ptr.

Re-add the null ptr check.

[1] https://gerrit-ring.savoirfairelinux.com/7156

Change-Id: Idfb502cd9b4c5b1e4a020b05120ce0dd21a014cf