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 56ac2cbd0464e0146e62c91843e2b1f5e0908504)
Signed-off-by: Diego Biurrun <diego@biurrun.de>

Conflicts:
	libavformat/apetag.c