demux: mp4: don't read at all if not content

fixes 1 byte read overflow

(cherry picked from commit 2d07e7885945ef30a55a078d956e4b6714c5c1ee)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mp4/libmp4.c

@@ -1396,7 +1396,7 @@ static int MP4_ReadBox_avcC( stream_t *p_stream, MP4_Box_t *p_box )
         if( !p_avcC->i_sps_length || !p_avcC->sps )
             goto error;
 
-        for( i = 0; i < p_avcC->i_sps && i_read; i++ )
+        for( i = 0; i < p_avcC->i_sps && i_read > 2; i++ )
         {
             MP4_GET2BYTES( p_avcC->i_sps_length[i] );
             if ( p_avcC->i_sps_length[i] > i_read )
@@ -1421,7 +1421,7 @@ static int MP4_ReadBox_avcC( stream_t *p_stream, MP4_Box_t *p_box )
         if( !p_avcC->i_pps_length || !p_avcC->pps )
             goto error;
 
-        for( i = 0; i < p_avcC->i_pps && i_read; i++ )
+        for( i = 0; i < p_avcC->i_pps && i_read > 2; i++ )
         {
             MP4_GET2BYTES( p_avcC->i_pps_length[i] );
             if( p_avcC->i_pps_length[i] > i_read )