* mu-msg-iter: add some casting to prevent compiler warning

src/mu-msg-iter.cc

@@ -225,7 +225,8 @@ get_field_number (MuMsgIter *iter, MuMsgFieldId mfid)
 unsigned int
 mu_msg_iter_get_docid (MuMsgIter *iter)
 {
-	g_return_val_if_fail (!mu_msg_iter_is_done(iter), -1);	
+	g_return_val_if_fail (!mu_msg_iter_is_done(iter),
+			      (unsigned int)-1);	
 	try {
 		return iter->_cursor.get_document().get_docid();
 

src/mu-msg-iter.h

@@ -82,7 +82,7 @@ MuMsg* mu_msg_iter_get_msg (MuMsgIter *iter, GError **err)
  * 
  * @param iter a valid MuMsgIter iterator
  * 
- * @return the docid or 0 in case of error
+ * @return the docid or (unsigned int)-1 in case of error
  */
 unsigned int     mu_msg_iter_get_docid         (MuMsgIter *iter);