dvb_get_channels is expected to append to an existing channel list.
For adapters supporting many delivery systems, a subsequent channel list
may turn up with a non-existent channel config, and the pointer
to the previously parsed channel list may be lost
(i.e. the list will be leaked and no channels detected).
Fix this by passing through the existing list (which may be NULL)
in case no new channels are found.
7021d095b1