Fix in blExtractZonePDBAsCopy()
blExtractZonePDBAsCopy() was core dumping if there was an exact match to the first residue in the file. 'prev' wasn't being set correctly to NULL.
blExtractZonePDBAsCopy() was core dumping if there was an exact match to the first residue in the file. 'prev' wasn't being set correctly to NULL.