visual: fix potential use-after-free

p_outpic is used to modify b_progressive before it is checked to be
non-null.

modules/visualization/visual/visual.c

@@ -340,9 +340,9 @@ static block_t *DoRealWork( filter_t *p_filter, block_t *p_in_buf )
 
     /* First, get a new picture */
     picture_t *p_outpic = vout_GetPicture( p_sys->p_vout );
-    p_outpic->b_progressive = true;
     if( unlikely(p_outpic == NULL) )
         return p_in_buf;
+    p_outpic->b_progressive = true;
 
     /* Blank the picture */
     for( int i = 0 ; i < p_outpic->i_planes ; i++ )