In AsyncSocket::handleErrMessages() inside the loop check if the callback was not...

In AsyncSocket::handleErrMessages() inside the loop check if the callback was not uninstalled

Summary: AsyncSocket::handleErrMessages() should check if the error message callback is still installing before calling it, since the callback could be uninstaled on the previous loop iteration.

Reviewed By: yfeldblum

Differential Revision: D5051001

fbshipit-source-id: fc01932c0d36bd8f72bf1905f12211fb83d28674