Fix output descriptors of c_{s,d,c,z}blat3

Fix output descriptors of c_{s,d,c,z}blat3

The NTRA argument can be equal to -1 if one does not want a snapshot file
(and this is the case with sample data {s,d,c,z}in3).
The routines {S,D,C,Z}PRCN3 will try to use their first argument as an output
unit number, so we avoid calling them when NTRA < 0.

Patch originally written by Camm Maguire.