BUG/MINOR: quic: fix crash on quic_conn alloc failure

If there is an alloc failure during qc_new_conn(), cleaning is done via
quic_conn_release(). However, since the below commit, an unchecked
dereferencing of <qc.path> is performed in the latter.

  e841164a44
  MINOR: quic: account for global congestion window

To fix this, simply check <qc.path> before dereferencing it in
quic_conn_release(). This is safe as it is properly initialized to NULL
on qc_new_conn() first stage.

This does not need to be backported.

src/quic_conn.c

@@ -1448,8 +1448,10 @@ int quic_conn_release(struct quic_conn *qc)
 	}
 
 	/* Substract last congestion window from global memory counter. */
+	if (qc->path) {
 		cshared_add(&quic_mem_diff, -qc->path->cwnd);
 		qc->path->cwnd = 0;
+	}
 
 	/* free remaining stream descriptors */
 	node = eb64_first(&qc->streams_by_id);