lavf/avio: check for : in filenames for protocols.

If the first "special" character in a filename is a comma,
it can introduce protocol options, but only if there is a
colon at the end. Otherwise, it is just a filename with a
comma.

Fix trac ticket #2303.
(cherry picked from commit d9fad53f4b447db1e436dcf3fc4a57e604616e6c)
Require at least three frames to autodetect loas.
(cherry picked from commit a60530e3ee1d9532c026a52b03661f88e163d647)