This silences some valgrind warnings.
CC: libav-stable@libav.org

Fixes second half of http://ffmpeg.org/trac/ffmpeg/ticket/794
Bug found by: Oana Stratulat
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Reinhard Tartler <siretart@tauware.de>
(cherry picked from commit f85334f5)