pgssubdec: reset rle_data_len/rle_remaining_len on allocation error

The code relies on their validity and otherwise can try to access a NULL
object->rle pointer, causing segmentation faults.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
