AVI: fix a crash in palette handling

This happen when p_vids->p_bih->biSize - sizeof( BITMAPINFOHEADER ) = 0
and the data is in p_vids->i_chunk_size, because then p_extra == NULL.

This solves the fruity sample crash but doesn't fix the palette

modules/demux/avi/avi.c

@@ -557,7 +557,7 @@ static int Open( vlc_object_t * p_this )
                 {
                     /* The palette is not always included in biSize */
                     fmt.i_extra = p_vids->i_chunk_size - sizeof(BITMAPINFOHEADER);
-                    if( fmt.i_extra > 0 )
+                    if( fmt.i_extra > 0 && fmt.p_extra )
                     {
                         const uint8_t *p_pal = fmt.p_extra;