MIDI clock start at the next beat (round_up_to_beat), so
here we have to round the current tick, rather than fall
back to a tick that is not yet complete, as 14da117bc8 does.
Reproduced with the Session from #9027
Start loop at bar 40 with MClk generator enabled.
```
#3 in __GI___assert_fail (assertion=0x7fedd86c4fd5 "clk_pos >= pos", file=0x7fedd86c38b7 "../libs/temporal/tempo.cc", line=3336, function=0x7fedd86c4f60 "void Temporal::TempoMap::midi_clock_beat_at_or_after(Temporal::samplepos_t, Temporal::samplepos_t&, uint32_t&) const") at assert.c:101
#4 in Temporal::TempoMap::midi_clock_beat_at_or_after(long, long&, unsigned int&) const (this= 0x560187e92c00, pos=20691033, clk_pos=@0x7fedc02178b8: 20691032, clk_beat=@0x7fedc02178c4: 11472) at ../libs/temporal/tempo.cc:3336
#5 in ARDOUR::MidiClockTicker::tick(long, long, unsigned int, long) (this=0x56018eed6db0, start_sample=20691033, end_sample=20692057, n_samples=1024, pre_roll=0) at ../libs/ardour/ticker.cc:170
#6 in ARDOUR::Session::send_mclk_for_cycle(long, long, unsigned int, long) (this=0x56018a216340, start_sample=20691033, end_sample=20692057, n_samples=1024, pre_roll=0) at ../libs/ardour/session.cc:7495
#7 in ARDOUR::AudioEngine::process_callback(unsigned int) (this=0x5601881a4f20, nframes=1024) at ../libs/ardour/audioengine.cc:563
```
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