[MU3] Fix #274126 Crash when entering music on top of an unterminated slur by MIDI

[lyrra]

Resolves: https://musescore.org/en/node/274126

Tiny fix, that enables different note-value entered from midi, on top of an slur.

• [x ] I signed CLA

Perhaps too late for 3.6.3, on the other hand, a very conservative change.

[Jojo-Schmitz]

I guess I'll just add it to #9000 (Edit: done), but won't it apply to master too?

[lyrra]

I guess I'll just add it to #9000 (Edit: done), but won't it apply to master too?
I guess it applies to master too. But I can't verify it in MU4, no audio/midi in gui.
MU3 is tested by launching two musescores and midi-connecting them in qjackctl graph.

[Jojo-Schmitz]

Yes, 3.x is often still pretty usefull as a testbed. In this case though, I'd just add this to master, no real need to test, the code is very obviously correct to protect from dereferencing a nullptr

[RomanPudashkin]

3.x is closed for any changes