This is more robust.

And only check if there is actually a frame returned.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit dd6c8575)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>