func_scramble: Fix null pointer dereference.

Fix segfault due to null pointer dereference
inside the audiohook callback.

ASTERISK-30220 #close

Change-Id: Ideb80f606974366e89d619d908744230b5a5a259

funcs/func_scramble.c

@@ -125,6 +125,7 @@ static int scramble_callback(struct ast_audiohook *audiohook, struct ast_channel
 	}
 
 	if (frame->frametype == AST_FRAME_VOICE) { /* only invert voice frequencies */
+		ni = datastore->data;
 		/* Based on direction of frame, and confirm it is applicable */
 		if (!(direction == AST_AUDIOHOOK_DIRECTION_READ ? ni->rx : ni->tx)) {
 			return 0;