Fixed fam->exec() condition

Check if the pointer is null - not a field of the struct

sprog.c

@@ -51,7 +51,7 @@ void sprog_process(const struct sprog_family *fam, void *arg, const struct sprog
   dup2(nstdin, 0);
   dup2(nstdout, 1);
   fam->init(arg);
-  if(d->data)
+  if(d)
     fam->exec(arg, d);
   dup2(oldstdin, 0);
   dup2(oldstdout, 1);