chan_sip: Check sip_pvt pointer in ast_channel_get_t38_state(c)

Asterisk may crash when calling ast_channel_get_t38_state(c)
on a locked channel which is being hung up.

ASTERISK-25609 #close

Change-Id: Ifaa707c04b865a290ffab719bd2e5c48ff667c7b

channels/chan_sip.c

@@ -4752,6 +4752,11 @@ static int sip_queryoption(struct ast_channel *chan, int option, void *data, int
 	struct sip_pvt *p = (struct sip_pvt *) ast_channel_tech_pvt(chan);
 	char *cp;
+	if (!p) {
+		ast_debug(1, "Attempt to Ref a null pointer. Sip private structure is gone!\n");
+		return -1;
+	}
+
 	sip_pvt_lock(p);
 	switch (option) {