The commit log message is somewhat unfortunate ;-)
The commit combines some cosmetic changes with a correction to see if there
is enough room for two extensions: renegotiation and SRTP.
In practice an overrun is not possible becaue at most a few bytes is written
to a BUF_MEM structure which is grown to 16k calling BUF_MEM_grow which
actually ends up allocating additional space making it 21848 bytes in length.