demux: mp4: fix buffer overflow in parsing of string boxes.

We ensure that pbox->i_size is never smaller than 8 to avoid an
integer underflow in the third argument of the subsequent call to
memcpy. We also make sure no truncation occurs when passing values
derived from the 64 bit integer p_box->i_size to arguments of malloc
and memcpy that may be 32 bit integers on 32 bit platforms.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mp4/libmp4.c

@@ -2858,6 +2858,9 @@ static int MP4_ReadBox_String( stream_t *p_stream, MP4_Box_t *p_box )
 {
     MP4_READBOX_ENTER( MP4_Box_data_string_t );
 
+    if( p_box->i_size < 8 || p_box->i_size > SIZE_MAX )
+        MP4_READBOX_EXIT( 0 );
+
     p_box->data.p_string->psz_text = malloc( p_box->i_size + 1 - 8 ); /* +\0, -name, -size */
     if( p_box->data.p_string->psz_text == NULL )
         MP4_READBOX_EXIT( 0 );