Move splitting vehicle move into horizontal and vertical into map::mo…

…ve_vehicle.

So the caller does not have to do it. At least one caller (from `monster::shove_vehicle`) did not do this.

src/map.cpp

@@ -434,6 +434,22 @@ static bool sees_veh( const Creature &c, vehicle &veh, bool force_recalc )
 
 vehicle *map::move_vehicle( vehicle &veh, const tripoint &dp, const tileray &facing )
 {
+    // Split the movement into horizontal and vertical for easier processing
+    if( dp.xy() != point_zero && dp.z != 0 ) {
+        vehicle *const new_pointer = move_vehicle( veh, tripoint( dp.xy(), 0 ), facing );
+        if( !new_pointer ) {
+            return nullptr;
+        }
+
+        vehicle *const result = move_vehicle( *new_pointer, tripoint( 0, 0, dp.z ), facing );
+        if( !result ) {
+            return nullptr;
+        }
+
+        result->is_falling = false;
+        return result;
+    }
+
     const bool vertical = dp.z != 0;
     if( ( dp.x == 0 && dp.y == 0 && dp.z == 0 ) ||
         ( abs( dp.x ) > 1 || abs( dp.y ) > 1 || abs( dp.z ) > 1 ) ||

src/vehicle_move.cpp

@@ -1335,18 +1335,7 @@ vehicle *vehicle::act_on_map()
         dp.z = -1;
     }
 
-    vehicle *new_pointer = this;
-    // Split the movement into horizontal and vertical for easier processing
-    if( dp.x != 0 || dp.y != 0 ) {
-        new_pointer = g->m.move_vehicle( *new_pointer, tripoint( dp.xy(), 0 ), mdir );
-    }
-
-    if( new_pointer != nullptr && dp.z != 0 ) {
-        new_pointer = g->m.move_vehicle( *new_pointer, tripoint( 0, 0, dp.z ), mdir );
-        is_falling = false;
-    }
-
-    return new_pointer;
+    return g->m.move_vehicle( *this, dp, mdir );
 }
 
 void vehicle::check_falling_or_floating()