isdn/gigaset: fix NULL pointer dereference

In do_action, a NULL pointer might be passed to function start_dial
which will dereference it.
Fix by adding a check for NULL before the call.

Spotted with Coverity.
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/gigaset/ev-layer.c

@@ -1380,6 +1380,11 @@ static void do_action(int action, struct cardstate *cs,
 	/* events from the LL */
 
 	case ACT_DIAL:
+		if (!ev->ptr) {
+			*p_genresp = 1;
+			*p_resp_code = RSP_ERROR;
+			break;
+		}
 		start_dial(at_state, ev->ptr, ev->parameter);
 		break;
 	case ACT_ACCEPT: