msg/async/ProtocolV2: short circuit empty segments and epilogue

msg/async/ProtocolV2: short circuit empty segments and epilogue

In both msgr2.0 and msgr2.1, segments can be empty.  In msgr2.1,
epilogue can be empty as well.

Handle both by calling the respective handler function directly
instead of allocating a buffer::ptr_node for an empty buffer and
passing that through READ[_RXBUF].

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit e6e3563d07181da043c0f8a7417c7a592af0c298)