We can't iterate over the object we are modifying

If subtract is called on itself, prefix and this is the same and the
iterator is invalid after modifying the underlaying object.

Instead just clear everyting.

Closes #354. Closes #355.

src/statesync/user.cc

@@ -43,9 +43,15 @@ using namespace ClientBuffers;
 
 void UserStream::subtract( const UserStream *prefix )
 {
+  // if we are subtracting ourself from ourself, just clear the deque
+  if ( this == prefix ) {
+    actions.clear();
+    return;
+  }
   for ( deque<UserEvent>::const_iterator i = prefix->actions.begin();
 	i != prefix->actions.end();
 	i++ ) {
+    assert( this != prefix );
     assert( !actions.empty() );
     assert( *i == actions.front() );
     actions.pop_front();