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>