Fix gcc warning: -Wuninitialized

/home/elsid/dev/openmw/apps/opencs/model/world/data.cpp: In constructor ‘CSMWorld::Data::Data(ToUTF8::FromType, bool, const Files::PathContainer&, const std::vector<std::__cxx11::basic_string<char> >&, const boost::filesystem::path&)’:
/home/elsid/dev/openmw/apps/opencs/model/world/data.cpp:69:36: warning: member ‘CSMWorld::Data::mCells’ is used uninitialized [-Wuninitialized]
   69 | : mEncoder (encoding), mPathgrids (mCells), mRefs (mCells),
      |                                    ^~~~~~

apps/opencs/model/world/data.hpp

@@ -98,13 +98,13 @@ namespace CSMWorld
             NestedIdCollection<ESM::Enchantment> mEnchantments;
             IdCollection<ESM::BodyPart> mBodyParts;
             IdCollection<ESM::MagicEffect> mMagicEffects;
-            SubCellCollection<Pathgrid> mPathgrids;
             IdCollection<ESM::DebugProfile> mDebugProfiles;
             IdCollection<ESM::SoundGenerator> mSoundGens;
             IdCollection<ESM::StartScript> mStartScripts;
             NestedInfoCollection mTopicInfos;
             InfoCollection mJournalInfos;
             NestedIdCollection<Cell> mCells;
+            SubCellCollection<Pathgrid> mPathgrids;
             IdCollection<LandTexture> mLandTextures;
             IdCollection<Land> mLand;
             RefIdCollection mReferenceables;