Reduce allocated length of the HTTP authentication request field buffer, as

noticed by Stefano and Luca in the "[PATCH]RTSP Basic Authentication"
mailinglist thread.

av_base64_encode() was recently changed. The previous implementation required
12 extra bytes (ceil(len(src)/3.)*4+12), whereas the new one is guaranteed to
fit in an exact buffer (ceil(len(src)/3.)*4), plus one extra byte for the
trailing zero. This change fixes no bug, it just slightly decreases the
amount of allocated memory.

Originally committed as revision 17761 to svn://svn.ffmpeg.org/ffmpeg/trunk