irssi: security bump to 0.8.21

irssi: security bump to 0.8.21

Bugfixes:

- CVE-2017-5193: Correct a NULL pointer dereference in the nickcmp function
  found by Joseph Bisch (GL#1)

- CVE-2017-5194: Correct an error when receiving invalid nick message (GL#4,
  #466)

- CVE-2017-5195: Correct an out of bounds read in certain incomplete control
  codes found by Joseph Bisch (GL#2)

- CVE-2017-5196: Correct an out of bounds read in certain incomplete
  character sequences found by Hanno Böck and independently by J.  Bisch
  (GL#3)

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>