libavformat: not treat 0 as EOF

transfer_func variable passed to retry_transfer_wrapper
are h->prot->url_read and h->prot->url_write functions.
These need to return EOF or other error properly.
In case of returning >= 0, url_read/url_write is retried
until error is returned.

Signed-off-by: Daniel Kucera <daniel.kucera@gmail.com>