libdvbv5: Allocate channel earlier

libdvbv5: Allocate channel earlier

As reported by Coverity:

CID 1228897 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)14. var_deref_model: Passing null pointer channel to get_program_and_store, which dereferences it. [show details]
1129                        rc = get_program_and_store(parms, *dvb_file, dvb_scan_handler,
1130                                                   service_id, channel, NULL,
1131                                                   get_detected, get_nit);

Well, get_program_and_store() explicitly tests if channel is null or
not, but it does it too late.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>