Use mutable instead of const_cast'ing this

Change-Id: I790cd140166bf0041abebbe0c4d9c15e931628f1

include/sot/stg.hxx

@@ -48,7 +48,7 @@ typedef struct SvGUID ClsId;
 class SOT_DLLPUBLIC StorageBase : public SvRefBase
 {
 protected:
-    sal_uLong       m_nError;                   // error code
+    mutable sal_uLong m_nError;                   // error code
     StreamMode      m_nMode;                    // open mode
     bool            m_bAutoCommit;
                     StorageBase();

sot/source/sdstor/stg.cxx

@@ -65,19 +65,19 @@ StorageBase::~StorageBase()
 sal_uLong StorageBase::GetError() const
 {
     sal_uLong n = m_nError;
-    const_cast<StorageBase*>(this)->m_nError = SVSTREAM_OK;
+    m_nError = SVSTREAM_OK;
     return n;
 }
 
 void StorageBase::SetError( sal_uLong n ) const
 {
     if( !m_nError )
-        const_cast<StorageBase*>(this)->m_nError = n;
+        m_nError = n;
 }
 
 void StorageBase::ResetError() const
 {
-    const_cast<StorageBase*>(this)->m_nError = SVSTREAM_OK;
+    m_nError = SVSTREAM_OK;
 }
 
 OLEStorageBase::OLEStorageBase( StgIo* p, StgDirEntry* pe, StreamMode& nMode )