This fixes a -Wunused-variable, see e.g
http://fate.ffmpeg.org/log.cgi?time=20150919162338&log=compile&slot=x86_64-archlinux-gcc-threads

.

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