[media] dvb-usb: improve sanity check of adap->active_fe in dvb_usb_ctrl_feed

The check for (adap->active_fe < 0) at the top of dvb_usb_ctrl_feed is a sanity
check to ensure that adap->active_fe is valid.  Improve that sanity check
by also checking for (adap->active_fe >= adap->num_frontends_initialized)

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>