vout_pictures: set pointer to NULL after release of picture data.

src/video_output/vout_pictures.c

@@ -1045,6 +1045,7 @@ void picture_Delete( picture_t *p_picture )
     free( p_picture->p_data_orig );
     free( p_picture->p_sys );
     free( p_picture );
+    p_picture = NULL;
 }
 
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