mds: do not assume segment list is non-empty in standby_trim_segments

If we restart standby replay shortly after startup, before we actually have
any segments, we an trigger a segfault here:

 ceph version 0.64-441-gc39b99c (c39b99cdecceaca77f66eafbcc38387406826406)
 1: ceph-mds() [0x975caa]
 2: (()+0xfcb0) [0x7fc33b5a5cb0]
 3: (MDLog::standby_trim_segments()+0x192) [0x78a932]
 4: (MDS::C_MDS_StandbyReplayRestartFinish::finish(int)+0x39) [0x595f69]
 5: (Journaler::_finish_reprobe(int, unsigned long, Context*)+0x190) [0x7917b0]
 6: (Filer::_probed(Filer::Probe*, object_t const&, unsigned long, utime_t)+0x558) [0x7c6b38]
 7: (Objecter::C_Stat::finish(int)+0xc0) [0x7c7930]
 8: (Objecter::handle_osd_op_reply(MOSDOpReply*)+0xe48) [0x7b2c78]
 9: (MDS::handle_core_message(Message*)+0xae8) [0x589858]
 10: (MDS::_dispatch(Message*)+0x2f) [0x589a1f]
 11: (MDS::ms_dispatch(Message*)+0x1d3) [0x58b4a3]
 12: (DispatchQueue::entry()+0x3f1) [0x943861]
 13: (DispatchQueue::DispatchThread::entry()+0xd) [0x86e32d]

Fixes: #5333
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>