ffmpeg: add return value check to supress the build warning.

add return value check to supress the build warning message like
"warning: ignoring return value" when use attribute -Wunused-result.

Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Reviewed-by: 刘歧 <lq@chinaffmpeg.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
