BUG/MEDIUM: mux_pt: dereference the connection with care in mux_pt_wake()

mux_pt_wake() calls data->wake() which can return -1 indicating that the
connection was just destroyed. We need to check for this condition and
immediately exit in this case otherwise we dereference a just freed
connection. Note that this mainly happens on idle connections between
two HTTP requests. It can have random implications between requests as
it may lead a wrong connection's polling to be re-enabled or disabled
for example, especially with threads.

This patch must be backported to 1.8.

src/mux_pt.c

@@ -51,6 +51,9 @@ static int mux_pt_wake(struct connection *conn)
 
 	ret = cs->data_cb->wake ? cs->data_cb->wake(cs) : 0;
 
+	if (ret < 0)
+		return ret;
+
 	/* If we had early data, and we're done with the handshake
 	 * then whe know the data are safe, and we can remove the flag.
 	 */