avcodec/movtextdec: Switch to smaller type

The base size of a box refers to the size the box has in a file,
not in memory; so size_t is not their natural type. Therefore use
a plain unsigned which is smaller on 64bit systems and still big
enough to represent any conceivable base size.

Reviewed-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
