Work around -fsanitize=null

(mpLayoutCache may be null, but that was harmless as Write was effectively static)

Change-Id: I41b10593c5ae98de92e7b305d9db75d0c4c7b334

sw/source/core/doc/docnew.cxx

@@ -797,7 +797,7 @@ void SwDoc::ReadLayoutCache( SvStream& rStream )
 
 void SwDoc::WriteLayoutCache( SvStream& rStream )
 {
-    mpLayoutCache->Write( rStream, *this );
+    SwLayoutCache::Write( rStream, *this );
 }
 
 IGrammarContact* getGrammarContact( const SwTxtNode& rTxtNode )

sw/source/core/inc/laycache.hxx

@@ -47,7 +47,7 @@ public:
     ~SwLayoutCache();
 
     void Read( SvStream &rStream );
-    void Write( SvStream &rStream, const SwDoc& rDoc );
+    static void Write( SvStream &rStream, const SwDoc& rDoc );
 
     void ClearImpl();
     bool IsLocked() const { return nLockCount > 0; }