Fix crash caused by repeated kicks of an already-kicked client

As it turns out, the QHash[] operator implicitly adds nullptrs to
the map when called with a nonexistent key.  If a user clicked the
button to kick another client multiple times, this would cause a
nullptr to be inserted into the peerMap, then causing a crash later
on at least when another client connected.  To solve the problem,
the value() method is used instead, which has no such side-effect.
A big thanks to @digitalcircuit for discovering this behavior.