#539 NULL pointer dereference in UPNP adress detection

[Superman666]

https://github.com/gtk-gnutella/gtk-gnutella/blob/efbb1d66d14d27c5f00d78f9891efa4aed60ccc4/src/upnp/upnp.c#L519

In upnp.c line 519 the variable sl is derefenced, but it's always NULL at this point that leads to a crash on my system:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff7fa88c0 (LWP 4386)]
upnp_discovered (devlist=0x7ffff7f08cd0, unused_arg=<optimized out="">) at upnp.c:519
519 selected = sl->data; / Pick the first /

[Raphael Manfredi]

Should be fixed by commit 416cfb26f689f5ea78e2bbbf60fe1fd68617acef