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.

CC: libav-stable@libav.org
Signed-off-by: Diego Biurrun <diego@biurrun.de>
(cherry picked from commit c5560e72)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>