make panel_hidden portable

fixing #325 while being a breaking change

pdcurses/panel.c

@@ -71,7 +71,8 @@ panel
 
    ceiling_panel() returns a pointer to the top panel in the deck.
 
-   panel_hidden() returns OK if pan is hidden and ERR if it is not.
+   panel_hidden() returns TRUE if pan is hidden, FALSE if not and
+   ERR if pan is NULL.
 
    panel_userptr() - Each panel has a user pointer available for
    maintaining relevant information. This function returns a pointer to
@@ -430,11 +431,10 @@ PANEL *ground_panel( SCREEN *sp)
 
 int panel_hidden(const PANEL *pan)
 {
-    assert( pan);
     if (!pan)
         return ERR;
 
-    return _panel_is_linked(pan) ? ERR : OK;
+    return _panel_is_linked(pan) ? TRUE : FALSE;
 }
 
 const void *panel_userptr(const PANEL *pan)