sw: assert that UnlockExpFlds() isn't called when it's not locked

Change-Id: Ibe2394978b776dd4a91a253856040fc45f81c7e9

sw/source/core/doc/DocumentFieldsManager.cxx

@@ -1207,6 +1207,7 @@ void DocumentFieldsManager::LockExpFlds()
 
 void DocumentFieldsManager::UnlockExpFlds()
 {
+    assert(mnLockExpFld != 0);
     if( mnLockExpFld )
         --mnLockExpFld;
 }