capmt: fix crash while trying to open iptv channel with capmt enabled

there is a crash in capmt.c while trying to watch an non-encrypted iptv channel

traceback: http://pastebin.com/Ucay9wz7

in general, capmt should be used ONLY with real dvb adapters, not iptv (no matter if it's encrypted or not)

this is a quick and dirty fix.