hfpmodem: Fix crash with more than two calls

A periodic CLCC polling is started when there is an ongoing multiparty
call and a new call appears in the system. A simple way to reproduce
the crashing scenario is:

1. Place a call.
2. Place a second call.
3. Create a multiparty call with both calls.
4. Place a third call (incoming or outgoing does not matter).
5. Disconnect HFP from the modem.

Within the function ciev_callheld_notify, the AT+CLCC command is also
invoked, thus a new cyclic CLCC polling is started, and it overwrites
the timer resource identifier stored in voicecall_data.clcc_source.
This means that there are several timers doing the CLCC polling, but
only one of those is under control, i.e. it can be removed through its
source identifier, hence a timer source leak.

This has a fatal consequence when the HFP modem is disconnected. The
function hfp_voicecall_remove stops the timer that is under control
before freeing the voicecall_data struct. However there are other timers
that are still active and will execute its handler poll_clcc afterwards.
Inside poll_clcc the driver_data is accessed, which is already NULL.

A solution for this is to avoid starting a CLCC polling if there is
already one active, i.e. clcc_source is not 0. By doing this the
uncontrolled timers will not cycle forever.

drivers/hfpmodem/voicecall.c

@@ -295,7 +295,7 @@ static void clcc_poll_cb(gboolean ok, GAtResult *result, gpointer user_data)
 	 * we won't get indicator update if any of them is released by CHLD=1x.
 	 * So we have to poll it.
 	 */
-	if (num_active > 1 || num_held > 1)
+	if ((num_active > 1 || num_held > 1) && !vd->clcc_source)
 		vd->clcc_source = g_timeout_add(POLL_CLCC_INTERVAL, poll_clcc,
 							vc);
 }