fix #281175: Removing rests attached to slurs in a voice >1 causes a crash

[Jojo-Schmitz]

fixes the symtom, the crash, and the cause, the orphaned slur, which gets removed now.
2nd part courtesy to @mattmcclinch

Still I guess it should go together with #4522, which fixes another related issue by removing an orphaned slur.

[mattmcclinch]

#4522 only removes slurs that were orphaned as a result of the implode function. There are other ways to orphan a slur, as described in https://musescore.org/en/node/281175.

[mattmcclinch]

This fixes the cause, in the case described in issue 281175: mattmcclinch@51eee8d.

[Jojo-Schmitz]

So do we need this instead of or in addition to mine?

[mattmcclinch]

In general, it is a good idea to check for nullptr before dereferencing a pointer. Besides, there may still be other ways to orphan a slur.

[ericfont]

Yes, both PRs should be accepted, as there almost surely are other ways which might produce orphaned slurs, such as: https://musescore.org/en/node/281083 "Slur should be deleted when changing duration of note deletes subsequent notes connected to the slur" which needs to be solved too.