Passing BytesIO object to av.open causes crash.

Created by: hhsprings

This code causes crash:

tmp = BytesIO()
# ... write bytes to tmp (for example wave format data) ...
container = av.open(tmp, "r")

We have to do seek(0) before calling open:

tmp = BytesIO()
# ... write bytes to tmp (for example wave format data) ...
tmp.seek(0)
container = av.open(tmp, "r")

Of course the fact that the result is not what we want without seek(0) is no problem, but crashing application is no good.