mac text renderer: Fix a nasty crash in the YUVA renderer

This really fixes #2649
This entire module needs some refinement, but since we are gonna have to dump it for CoreText very soon, there is little point in doing that.
(cherry picked from commit ee313399)
Signed-off-by: Derk-Jan Hartman <hartman@videolan.org>

modules/misc/quartztext.c

@@ -864,10 +864,10 @@ static int RenderYUVA( filter_t *p_filter, subpicture_region_t *p_region, UniCha
     p_dst_a = p_region->p_picture->A_PIXELS;
     i_pitch = p_region->p_picture->A_PITCH;
 
-    i_offset = VERTICAL_MARGIN *i_pitch;
-    for( y=0; y<i_textblock_height; y++)
+    i_offset = (i_height+VERTICAL_MARGIN < fmt.i_height) ? VERTICAL_MARGIN *i_pitch : 0 ;
+    for( y=0; y<fmt.i_height; y++)
     {
-        for( x=0; x<i_width; x++)
+        for( x=0; x<fmt.i_width; x++)
         {
             int i_alpha = p_offScreen->p_data[ y * p_offScreen->i_bytesPerRow + x * p_offScreen->i_bytesPerPixel     ];
             int i_red   = p_offScreen->p_data[ y * p_offScreen->i_bytesPerRow + x * p_offScreen->i_bytesPerPixel + 1 ];