Fix crash when persistent data refers to missing buildings. Can only happen if planned buildings are removed with the plugin removed, then the game is reloaded after the plugin is put back.

plugins/buildingplan.cpp

@@ -37,7 +37,7 @@ using df::global::ui_build_selector;
 using df::global::world;
 
 DFHACK_PLUGIN("buildingplan");
-#define PLUGIN_VERSION 0.7
+#define PLUGIN_VERSION 0.8
 
 #ifndef HAVE_NULLPTR
 #define nullptr 0L
@@ -496,6 +496,9 @@ public:
         }
 
         building = df::building::find(config.ival(1));
+        if (!building)
+            return;
+
         pos = df::coord(building->centerx, building->centery, building->z);
         filter.min_quality = static_cast<df::item_quality>(config.ival(2) - 1);
         filter.decorated_only = config.ival(3) - 1;