Summary:
It's possible that AsyncTransport read completed successfully in one loop, then the read coro gets a cancellation request.  In this case, prefer to return successfully with the read data.

Read cancellation need not be fatal, eg: an application may just want to interrupt a read and come back to it later.  In this case, the successfully read data could be lost.

In particular, if an EOF was lost, the application might start another read, AsyncSocket::setReadCB can assert in invalidState(ReadCallback*) because the DestructorGuard count is 0.

Reviewed By: yairgott

Differential Revision: D30964504

fbshipit-source-id: ea963e6572f2bb552899d3b1e6274bbd0bcdc265