webrtcbin: Fixed memory leak in gstwebrtcstats
The function _get_stats_from_ice_transport returns a string which must be freed by the caller. However, _get_stats_from_dtls_transport was ignoring the return value from this function, resulting in a leak.
Ran this with valgrind. Before this fix there was a leak of 40 bytes each time this was called. After there was no leak.