Fix "invalid" activity zones not spawning

There's a problem in #1224 where it breaks spawning custom activity
zones. After a bit of confusion after this was reported, I realized
that I removed the fallback from `getcrewman` and changed the return
value to `-1` if the entity isn't found, to avoid returning the wrong
entity (entity 0, aka probably the player).

Unfortunately, it seems like a ton of levels (including my older ones)
rely on this behavior.

Creating custom activity zones is a long process which uses a bunch of
unintended behaviour, which includes targeting a crewmate with color
35. With the change I mentioned earlier, the `getcrewman` function
would return `-1` instead, which was out of bounds of the entity array,
so the game avoided spawning the activity zone at all. The prior
behaviour of falling back to entity 0 (most likely the player) would
spawn the activity zone around the player instead.

Nowadays, I try to spawn a crewmate with color 35 anyways so I can
control where the box spawns (instead of on the player always), however
most people don't (and haven't) so reverting this change seems best for
now.

If we wanted to reintroduce the `-1` fallback in the future, things
that call `getcrewman` would have to check for `-1` and use `0`
instead, but that would require a lot more testing and studying where
it's used, and I'd rather squash this bug quickly and worry about
cleanliness later.

desktop_version/src/Entity.cpp

@@ -4011,7 +4011,11 @@ int entityclass::getcrewman(int t)
         }
     }
 
-    return -1;
+    // Return entity 0 as a fallback
+    // Unfortunately some levels rely on this, where targeting a non-existent crewman returns the first entity...
+    // Which, most of the time, is the player.
+
+    return 0;
 }
 
 int entityclass::getcustomcrewman(int t)