SICS-648: memory leak in nxscript.c:putslab

fix memory leak (but only on malloc path)
Originally committed on SVN as,
r2428 | dcl | 2008-04-14 09:47:50 +1000 (Mon, 14 Apr 2008)

nxscript.c

@@ -1008,6 +1008,8 @@ static void putSlab(SConnection * pCon, SicsInterp * pSics, pNXScript self,
       if (status == NX_OK) {
         written = 1;
       }
+      if (argc > 6)
+        free(histData);
     }
   }