Misc changes.

The first two fixes were caught by infer. The Big Bug(tm) remains
unfixed.

* Fixed the Leech nano power
* Fixed an unlikely nullptr dereference in ItemManager
* /toggleai now makes mobs retreat immediately, instead of waiting for
their next movement tick
* Static path mobs will now stop in place instead of retreating to
their spawn points when AI is disabled
* Changed the misleading config option name from "chunksize" to
"viewdistance", since it's actually only a third of the chunk size

src/settings.hpp

@@ -8,7 +8,7 @@ namespace settings {
     extern int SHARDPORT;
     extern std::string SHARDSERVERIP;
     extern time_t TIMEOUT;
-    extern int CHUNKSIZE;
+    extern int VIEWDISTANCE;
     extern bool SIMULATEMOBS;
     extern int SPAWN_X;
     extern int SPAWN_Y;