[Minor] Fix a conflict between CanBeBuiltOn logic and UndeploysInto unhardcoding (#1819)

Fix the issue of building with `UndeploysInto` getting stuck when
placing/deploying to a place that the top cell has an object with
`IsPassable=false` and `CanBeBuiltOn=true` since the `WaterBound=true`
buildings' `UndeploysInto` fix.

Author: CrimRecya

src/Misc/Hooks.BugFixes.cpp

@@ -1625,8 +1625,15 @@ DEFINE_HOOK(0x446D42, BuildingClass_Place_FreeUnit_NearByLocation2, 0x6)
 
 DEFINE_HOOK(0x449462, BuildingClass_IsCellOccupied_UndeploysInto, 0x6)
 {
-	enum { SkipGameCode = 0x449487 };
+	enum { PlacingCheck = 0x449493, SkipGameCode = 0x449487 };
 
+	GET(BuildingClass*, pThis, ECX);
+
+	// Placing check, only newly generated buildings in this mission
+	if (pThis->CurrentMission == Mission::None)
+		return PlacingCheck;
+
+	// Undeploying check
 	GET(BuildingTypeClass*, pType, EAX);
 	LEA_STACK(CellStruct*, pDest, 0x4);
 	const auto pCell = MapClass::Instance.GetCellAt(*pDest);