[MusicXML import] invalid step not handled

The MusicXML file attached to https://musescore.org/en/node/328982 results in a failed assertion when imported using a debug build of MuseScore. Root cause is an invalid step at line 18985.

Stack trace:
0 libsystem_kernel.dylib 0x00007fff5323bb66 __pthread_kill + 10
1 libsystem_pthread.dylib 0x00007fff53406080 pthread_kill + 333
2 libsystem_c.dylib 0x00007fff531971ae abort + 127
3 org.qt-project.QtCore 0x000000011062c749 qt_message_fatal(QtMsgType, QMessageLogContext const&, QString const&) + 9
4 org.qt-project.QtCore 0x000000011062e0c0 QMessageLogger::fatal(char const*, ...) const + 224
5 org.qt-project.QtCore 0x0000000110629248 qt_assert(char const*, char const*, int) + 72
6 org.musescore.MuseScore 0x0000000107769cad Ms::step2tpc(int, Ms::AccidentalVal) + 77
7 org.musescore.MuseScore 0x00000001073f9f03 Ms::xmlSetPitch(Ms::Note*, int, int, int, int, Ms::Instrument const) + 115
8 org.musescore.MuseScore 0x00000001073e2537 Ms::setPitch(Ms::Note, Ms::MusicXMLParserPass1&, QString const&, QString const&, Ms::mxmlNotePitch const&, int, Ms::Instrument const) + 631
9 org.musescore.MuseScore 0x00000001073cb856 Ms::MusicXMLParserPass2::note(QString const&, Ms::Measure, Ms::Fraction, Ms::Fraction, Ms::Fraction&, Ms::Frac ...

Relates to #270643: [EPIC] MusicXML import/export issues.