BUG/MAJOR: stream: avoid double free on unique_id

BUG/MAJOR: stream: avoid double free on unique_id

Commit 32211a1 ("BUG/MEDIUM: stream: Don't forget to free
s->unique_id in stream_free().") addressed a memory leak but in
exchange may cause double-free due to the fact that after freeing
s->unique_id it doesn't null it and then calls http_end_txn()
which frees it again. Thus the process quickly crashes at runtime.

This fix must be backported to all stable branches where the
aforementioned patch was backported.