[starsim] Fix logic handling multiple hit banks in the stgc (#499)

g2t_stg.F copies hits from the three sensitive volumes into the single
g2t_stg_hit table.
When no hits are registered in a single sensitive volume, the loop
_should_ proceed to
query the next SV for hits. Instead, the code branched to the return
statement, resulting
in no hits being saved for the remaining SVs.  

This PR fixes the issue.

Author: klendathu2k

pams/sim/g2t/g2t_stg.F

@@ -20,11 +20,10 @@ FUNCTION  G2T_STG (g2t_track_h,   g2t_track ,
            if (isys.eq.1) CALL g2r_get_sys('STGM','STGP',Iprin,Idigi)
            if (isys.eq.2) CALL g2r_get_sys('STGM','STGL',Iprin,Idigi)
            if (isys.eq.3) CALL g2r_get_sys('STGM','STGS',Iprin,Idigi)
-           IF (Iprin.lt.0) go to 99
 
-           
+           IF (Iprin.ge.0 ) THEN
 
-           Do While (G2R_GET_HIT('stg') .eq. 0)
+              Do While (G2R_GET_HIT('stg') .eq. 0)
 
                  i=i+1
                  g2t_fts_hit_h.nok            = i
@@ -49,7 +48,10 @@ FUNCTION  G2T_STG (g2t_track_h,   g2t_track ,
                  g2t_track(trac).hit_fts_p    = i
                  g2t_track(trac).n_stg_hit    = g2t_track(trac).n_stg_hit + 1
 
-           enddo * G2R_GET_HIT
+              enddo * G2R_GET_HIT
+
+           ENDIF
+
       enddo * Isys
 
  99   RETURN