Log Message:
irssi: update to 0.8.21.

irssi 0.8.21 is a maintenance release without any new features.

Changes:

- Correct a NULL pointer dereference in the nickcmp function found by
Joseph Bisch (GL#1)
- Correct an out of bounds read in certain incomplete control codes
found by Joseph Bisch (GL#2)
- Correct an out of bounds read in certain incomplete character
sequences found by Hanno Böck and independently by J. Bisch (GL#3)
- Correct an error when receiving invalid nick message (GL#4, #466)