quassel: 0.12.4 -> 0.12.5 (fixes RCE & remote crash)

It was found that Quassel could be remotely crashed and had an
unauthenticated RCE vulnerability. The public annoucement can be found
on the oss-sec archive [1]. The bump to 0.12.5 is supposed fixe both issues.

[1] http://seclists.org/oss-sec/2018/q2/77