[MusicXML import] abort while importing BeetAnGeSample.xml

Todays trunk aborts while reading Recordares sample file BeetAnGeSample.xml.

Note that the MuseScore test files containing slurs (testSlurs.xml and testSlurTieLineStyle.xml) import without errors.

Relevant part of stack trace:

#4 0x0000000101bbddb1 in QMessageLogger::fatal ()
#5 0x00000001007eb25a in Ms::ChordRest::add (this=0x117a86fd0, e=0x117a87520) at /Users/lvi/dev/MuseScore/libmscore/chordrest.cpp:944
#6 0x00000001007b429d in Ms::Chord::add (this=0x117a86fd0, e=0x117a87520) at /Users/lvi/dev/MuseScore/libmscore/chord.cpp:477
#7 0x000000010093dc14 in Ms::Score::addElement (this=0x1080fda00, element=0x117a87520) at /Users/lvi/dev/MuseScore/libmscore/score.cpp:1328
#8 0x000000010033c2aa in Ms::MusicXml::xmlNotations (this=0x7fff5fbf9658, note=0x117a871c0, cr=0x117a86fd0, trk=4, ticks=0, e=@0x7fff5fbf7380) at /Users/lvi/dev/MuseScore/mscore/importxml.cpp:4179
#9 0x000000010032c24c in Ms::MusicXml::xmlNote (this=0x7fff5fbf9658, measure=0x117a5b510, staff=1, partId=@0x7fff5fbf85e8, beam=@0x7fff5fbf86b0, currentVoice=@0x7fff5fbf8628, e=@0x7fff5fbf85e0, graceNotes=@0x7fff5fbf8620, alt=@0x7fff5fbf86e0) at /Users/lvi/dev/MuseScore/mscore/importxml.cpp:5166
#10 0x0000000100320ca4 in Ms::MusicXml::xmlMeasure (this=0x7fff5fbf9658, part=0x117a47ac0, e=@0x7fff5fbf8a18, number=8, measureLen={_numerator = 3, _denominator = 4}, currKeySig=0x7fff5fbf8a48) at /Users/lvi/dev/MuseScore/mscore/importxml.cpp:2058