prefer avio_check() over url_exist()

The problem with url_exist() is that it tries to open a resource in
RDONLY mode. If the file is a FIFO and there is already a reading
client, the open() call will hang.

By using avio_check() with access mode of 0, the second reading
process will check if the file exists without attempting to open it,
thus avoiding the lock.

Fix issue #1663.

Signed-off-by: Anton Khirnov <anton@khirnov.net>