Fix index error if lazy_load_members but no events

[erdnaxeli]

I got the following IndexError:

2018-12-28 13:28:58,712 - synapse.http.server - 112 - ERROR - GET-565- Failed handle request via <function _async_render at 0x7f22fa949a28>: <XForwardedForRequest at 0x7f22acf08ef0 method=u'GET' uri=u'/_matrix/client/r0/rooms/!hUXVblRFCyynXMImeV:iiens.net/messages?from=t133592-3043003_41918178_53053_1733512_446400_976_1490_178292_19&dir=b&limit=30&filter=%7B%22lazy_load_members%22%3Atrue%7D' clientproto=u'HTTP/1.1' site=8008>: Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 653, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1442, in gotResult
    _inlineCallbacks(r, g, deferred)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/synapse/http/server.py", line 81, in wrapped_request_handler
    yield h(self, request)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python2.7/dist-packages/synapse/http/server.py", line 316, in _async_render
    callback_return = yield callback(request, **kwargs)
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python2.7/dist-packages/synapse/rest/client/v1/room.py", line 479, in on_GET
    event_filter=event_filter,
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1386, in _inlineCallbacks
    result = g.send(result)
  File "/usr/lib/python2.7/dist-packages/synapse/handlers/pagination.py", line 266, in get_messages
    events[0].event_id, state_filter=state_filter,
exceptions.IndexError: list index out of range

This PR fixes it.

Signed-off-by: Alexandre Morignot erdnaxeli@gmail.com

[erdnaxeli]

Duplicate of #4330