webrtc: Fix segfault traversing ice transports

webrtc: Fix segfault traversing ice transports

Previously, reassigning loop index l in nicestream.c
could cause a segfault if l->data was null, as it could
reassign l to a null variable, triggering the loop
postassignment l->next, which then segfaults due to
l now being null. It is instead moved into the loop.
_delete_transport already performs the reassignment
inline.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4213>