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 d14696c99ccac12a052ce10e70859ffc0293ed6a)
Signed-off-by: Diego Biurrun <diego@biurrun.de>