Use dynamic_cast for safety in eeconfig()

Calling config() gets you whatever APP_SETTINGS_BASE is returned
by the underlying kiface, which is currently going to be an
EESCHEMA_SETTINGS everywhere this is used, but that isn't a
guarantee.

eeschema/sch_base_frame.h

@@ -114,7 +114,7 @@ public:
     SCH_SCREEN* GetScreen() const override;
     void SetScreen( BASE_SCREEN* aScreen ) override;
 
-    EESCHEMA_SETTINGS* eeconfig() const { return static_cast<EESCHEMA_SETTINGS*>( config() ); }
+    EESCHEMA_SETTINGS* eeconfig() const { return dynamic_cast<EESCHEMA_SETTINGS*>( config() ); }
 
     void LoadSettings( APP_SETTINGS_BASE* aCfg ) override;
     void SaveSettings( APP_SETTINGS_BASE* aCfg ) override;