Because the lavf_container is sometimes called with only 2 arguments,
fate tests produce bash errors like this:

  tests/fate-run.sh: 299: test: =: unexpected operator

This commit fixes this.
Reviewed-by: Limin Wang <lance.lmwang@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>