Only initialize steam-engine when a fortress mode map is loaded

world->raws.buildings.workshops is not always cleared on world load
(notably in the arena), so it can contain invalid pointers from
previous games.

Fixes #444

NEWS

@@ -8,6 +8,7 @@ DFHack Future
     New tweaks
     New features
     Fixes
+        steam-engine: fixed a crash on arena load
     Misc Improvements
     Removed
 

plugins/steam-engine.cpp

@@ -5,6 +5,7 @@
 #include <modules/Gui.h>
 #include <modules/Screen.h>
 #include <modules/Maps.h>
+#include <modules/World.h>
 #include <TileTypes.h>
 #include <vector>
 #include <cstdio>
@@ -978,8 +979,8 @@ static void enable_hooks(bool enable)
 DFhackCExport command_result plugin_onstatechange(color_ostream &out, state_change_event event)
 {
     switch (event) {
-    case SC_WORLD_LOADED:
-        if (find_engines(out))
+    case SC_MAP_LOADED:
+        if (World::isFortressMode() && find_engines(out))
         {
             out.print("Detected steam engine workshops - enabling plugin.\n");
             enable_hooks(true);
@@ -987,7 +988,7 @@ DFhackCExport command_result plugin_onstatechange(color_ostream &out, state_chan
         else
             enable_hooks(false);
         break;
-    case SC_WORLD_UNLOADED:
+    case SC_MAP_UNLOADED:
         enable_hooks(false);
         engines.clear();
         break;