message: Fix crash when application is using unsupported function.
  It fixes this crash:
diff: @@ -1011,8 +1011,12 @@ void WrappedOpenGL::UseUnusedSupportedFunction(const char *name)
  if(it->second.Modern())
  {
  RenderDoc::Inst().RemoveDeviceFrameCapturer(it->second.ctx);
  - for(auto wnd : it->second.windows)
  - it->second.UnassociateWindow(this, wnd.first);
  + for(auto wnd = it->second.windows.begin(); wnd != it->second.windows.end();)
  + {
  + void *wndHandle = wnd->first;
  + wnd++;
  + it->second.UnassociateWindow(this, wndHandle);
  + }
  }
  }
  }
  