Add more configuration parameter lower-bound checks.

netslide.c

@@ -318,6 +318,8 @@ static const char *validate_params(const game_params *params, bool full)
 	return "Barrier probability may not be negative";
     if (params->barrier_probability > 1)
 	return "Barrier probability may not be greater than 1";
+    if (params->movetarget < 0)
+        return "Number of shuffling moves may not be negative";
     return NULL;
 }