Don't cache incorrect length in wxString::DoUngetWriteBuf().

The length here is the size of the buffer in bytes and is not necessarily the
string length in code points (notably not for non-ASCII strings in UTF-8
build).

Closes #14130.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70988 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/string.h

@@ -3485,7 +3485,7 @@ private:
 
   void DoUngetWriteBuf(size_t nLen)
   {
-      wxSTRING_SET_CACHED_LENGTH(nLen);
+      wxSTRING_INVALIDATE_CACHE();
 
       m_impl.DoUngetWriteBuf(nLen);
   }