[MusicXML Export] crash exporting Telemann sample as MusicXML

Current trunk segfaults when exporting Recordares Telemann sample as MusicXML. No crash when saving in MuseScore format. Impact unknown, don't yet know what triggers it or how likely it is. may or may not be critical.

Partial stack trace:
#0 Ms::Segment::segmentType (this=0x0) at segment.h:171
#1 0x00000001001d379b in Ms::hasBreathMark (ch=0x110562de0) at /Users/lvi/dev/MuseScore/mscore/exportxml.cpp:1691
#2 0x00000001001d2480 in Ms::ExportMusicXml::chordAttributes (this=0x7fff5fbfb3e0, chord=0x110562de0, notations=@0x7fff5fbf9e18, technical=@0x7fff5fbf9e10, trillStart=@0x7fff5fbfb5b8, trillStop=@0x7fff5fbfb5c0) at /Users/lvi/dev/MuseScore/mscore/exportxml.cpp:1863
#3 0x00000001001d7107 in Ms::ExportMusicXml::chord (this=0x7fff5fbfb3e0, chord=0x110562de0, staff=1, ll=0x110562ed0, useDrumset=Ms::NONE) at /Users/lvi/dev/MuseScore/mscore/exportxml.cpp:2528
#4 0x00000001001e47b7 in Ms::ExportMusicXml::write (this=0x7fff5fbfb3e0, dev=0x7fff5fbfb5e8) at /Users/lvi/dev/MuseScore/mscore/exportxml.cpp:4754