apetag: Fix APE tag size check

The size variable is (correctly) unsigned, but is passed to several functions
which take signed parameters, such as avio_read, sometimes after having
numbers added to it. So ensure that size remains within the bounds that
these functions can handle.

(cherry picked from commit c5560e72d0bb69f8a1ac9536570398f84388f396)
Signed-off-by: Diego Biurrun <diego@biurrun.de>