Free zstring when switching to preset value

This is only applicable for bounded cvars (MIN, MAX),
since otherwise there's no way to allocate a zstring.

src/command.c

@@ -1393,6 +1393,10 @@ static void Setvalue(consvar_t *var, const char *valstr, boolean stealth)
 						return;
 					}
 
+					// free the old value string
+					Z_Free(var->zstring);
+					var->zstring = NULL;
+
 					var->value = var->PossibleValue[i].value;
 					var->string = var->PossibleValue[i].strvalue;
 					goto finish;