free multi-level construction correctly, avoid duplication

src/kernel/build.c

@@ -977,3 +977,12 @@ void continue_ship(unit * u, int want)
     build_ship(u, sh, want);
 }
 
+void free_construction(struct construction *cons)
+{
+    while (cons) {
+        construction *next = cons->improvement;
+        free(cons->materials);
+        free(cons);
+        cons = next;
+    }
+}

src/kernel/build.h

@@ -65,6 +65,7 @@ extern "C" {
 
     } construction;
 
+    void free_construction(struct construction *cons);
     extern int destroy_cmd(struct unit *u, struct order *ord);
     extern int leave_cmd(struct unit *u, struct order *ord);
 

src/kernel/building.c

@@ -94,10 +94,7 @@ void bt_register(building_type * type)
 
 void free_buildingtype(void *ptr) {
     building_type *btype = (building_type *)ptr;
-    if (btype->construction) {
-        free(btype->construction->materials);
-        free(btype->construction);
-    }
+    free_construction(btype->construction);
     free(btype->_name);
     free(btype);
 }

src/kernel/ship.c

@@ -245,10 +245,7 @@ static void free_shiptype(void *ptr) {
     ship_type *stype = (ship_type *)ptr;
     free(stype->_name);
     free(stype->coasts);
-    if (stype->construction) {
-        free(stype->construction->materials);
-        free(stype->construction);
-    }
+    free_construction(stype->construction);
     free(stype);
 }