video_chroma:copy: fix the number of lines for the second NV12 planes

The pitch is similar to the full picture, but there's half the lines.

Compared to I420 where the second/third planes have half width and half height.

https://hackerone.com/reports/485437

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>
(cherry picked from commit eeb662bca2)
Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

modules/video_chroma/copy.c

@@ -969,7 +969,7 @@ int picture_UpdatePlanes(picture_t *picture, uint8_t *data, unsigned pitch)
 
             p->p_pixels = o->p_pixels + o->i_lines * o->i_pitch;
             p->i_pitch  = pitch;
-            p->i_lines  = picture->format.i_height;
+            p->i_lines  = picture->format.i_height / 2;
             assert(p->i_visible_pitch <= p->i_pitch);
             assert(p->i_visible_lines <= p->i_lines);
         }