Fix Forestry in growing zones (#113)

Missing key in dictionary? Check if key exists first.

Author: alextd

Source/ManagerJobs/ManagerJob_Forestry.cs

@@ -205,7 +205,7 @@ public void DoClearAreaDesignations( IEnumerable<IntVec3> cells, ref bool workDo
                     continue;
 
                 // if the plant is not in the allowed filter
-                if ( !AllowedTrees[plant.def] )
+                if (!AllowedTrees.ContainsKey(plant.def) ||  !AllowedTrees[plant.def] )
                     continue;
 
                 // we don't cut stuff in growing zones