This fixes a bug/regression with very small packets
Fixes: output_file

Regression since: 0782fb6bReported-by: Thierry Foucu <tfoucu@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d25c9452)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>