Previously, the following syntax elements were not read in the case
sps_num_subpics_minus is 0:
* sps_subpic_id_len_minus1
* sps_subpic_id_mapping_explicitly_signalled_flag
* sps_subpic_id_mapping_present_flag
* sps_subpic_id[i]
This was causing failures to decode bitstreams, for example the DVB's
"VVC HDR UHDTV1 OpenGOP 3840x2160 50fps HLG10 PiP" V&V bitstream.

Patch fixes this by moving the reads for these syntax elements out a
scope.
Signed-off-by: Frank Plowman <post@frankplowman.com>
Signed-off-by: James Almer <jamrial@gmail.com>