res_stasis_snoop: Fix snoop crash

Added NULL pointer check and channel lock to prevent resource release
while the chanspy is processing.

ASTERISK-29604

Change-Id: Ibdc675f98052da32333b19685b1708a3751b6d24

res/res_stasis_snoop.c

@@ -135,10 +135,14 @@ static void publish_chanspy_message(struct stasis_app_snoop *snoop, int start)
 	}
 	ast_multi_channel_blob_add_channel(payload, "spyer_channel", snoop_snapshot);
 
+	if (snoop->spyee_chan) {
+		ast_channel_lock(snoop->spyee_chan);
 		spyee_snapshot = ast_channel_snapshot_get_latest(ast_channel_uniqueid(snoop->spyee_chan));
+		ast_channel_unlock(snoop->spyee_chan);
 		if (spyee_snapshot) {
 			ast_multi_channel_blob_add_channel(payload, "spyee_channel", spyee_snapshot);
 		}
+	}
 
 	message = stasis_message_create(type, payload);
 	if (!message) {