libmp4: minor bug fix

If we read all the stream but n bytes, 0 < n < 8, there's no point to
try and read these n missing bytes.

If we try to continue the parsing, MP4_ReadBoxCommon() would return 0,
and VLC would fall back to avformat's demux while it is not necessary.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mp4/libmp4.c

@@ -3571,7 +3571,7 @@ MP4_Box_t *MP4_BoxGetRoot( stream_t *s )
         return p_root;
 
     p_root->i_size = stream_Size( s );
-    if( stream_Tell( s ) < stream_Size( s ) )
+    if( stream_Tell( s ) + 8 < stream_Size( s ) )
     {
         /* Get the rest of the file */
         i_result = MP4_ReadBoxContainerRaw( p_stream, p_root );