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>