avcodec/apedec: fix undefined left shifts of negative numbers

This fixes -Wshift-negative-value reported with clang 3.7+, e.g
http://fate.ffmpeg.org/log.cgi?time=20150919172459&log=compile&slot=x86_64-darwin-clang-polly-notiling-3.7.
Note that the patch crucially depends on int >= 32 bits,
an assumption made in many places in the codebase.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
