Correct the length of base64 digests

This was broken in e42b5d09. This makes adobe auth against
akamai servers work again.
Only use one challenge value in adobe auth

Some servers (akamai) only include a 'challenge' parameter, no
'opaque' parameter, while others (wowza) include both 'opaque'
and 'challenge'. If 'opaque' is provided, it should be used here,
otherwise 'challenge', but never both.

After this patch, adobe auth works against both wowza and akamai
servers.
Remove a useless leftover comment

The variable is any random challenge, thus the byte order doesn't
matter at all.
Merge branch 'master' of git://git.ffmpeg.org/rtmpdump