Add missing p_vout->pf_end in vmem.c and snapshot.c

The p_vout->pf_end method is mandatory for video output modules. It is
called from so many places in libvlc that it would be too tedious to
make it optional. I'm therefore adding empty methods to modules instead.

modules/video_output/snapshot.c

@@ -55,6 +55,7 @@ static int  Create    ( vlc_object_t * );
 static void Destroy   ( vlc_object_t * );
 
 static int  Init      ( vout_thread_t * );
+static void End       ( vout_thread_t * );
 static void Display   ( vout_thread_t *, picture_t * );
 
 /*****************************************************************************
@@ -123,7 +124,7 @@ static int Create( vlc_object_t *p_this )
     var_Create( p_vout, "snapshot-list-pointer", VLC_VAR_ADDRESS );
 
     p_vout->pf_init = Init;
-    p_vout->pf_end = NULL;
+    p_vout->pf_end = End;
     p_vout->pf_manage = NULL;
     p_vout->pf_render = NULL;
     p_vout->pf_display = Display;
@@ -317,6 +318,14 @@ static int Init( vout_thread_t *p_vout )
     return VLC_SUCCESS;
 }
 
+/*****************************************************************************
+ * End: terminate video thread output method
+ *****************************************************************************/
+static void End( vout_thread_t *p_vout )
+{
+    (void)p_vout;
+}
+
 /*****************************************************************************
  * Destroy: destroy video thread
  *****************************************************************************

modules/video_output/vmem.c

@@ -39,6 +39,7 @@ static int  Create    ( vlc_object_t * );
 static void Destroy   ( vlc_object_t * );
 
 static int  Init          ( vout_thread_t * );
+static void End           ( vout_thread_t * );
 static int  LockPicture   ( vout_thread_t *, picture_t * );
 static int  UnlockPicture ( vout_thread_t *, picture_t * );
 
@@ -115,7 +116,7 @@ static int Create( vlc_object_t *p_this )
         return VLC_ENOMEM;
 
     p_vout->pf_init = Init;
-    p_vout->pf_end = NULL;
+    p_vout->pf_end = End;
     p_vout->pf_manage = NULL;
     p_vout->pf_render = NULL;
     p_vout->pf_display = NULL;
@@ -249,6 +250,14 @@ static int Init( vout_thread_t *p_vout )
     return VLC_SUCCESS;
 }
 
+/*****************************************************************************
+ * End: terminate video thread output method
+ *****************************************************************************/
+static void End( vout_thread_t *p_vout )
+{
+    (void)p_vout;
+}
+
 /*****************************************************************************
  * Destroy: destroy video thread
  *****************************************************************************