fix #274028: Courtesy accidental on tied note ignored later in measure

[mattmcclinch]

See https://musescore.org/en/node/274028.

[mattmcclinch]

It occurs to me that checking the role of the accidental is probably unnecessary, and is potentially problematic, should MuseScore ever automatically add a courtesy accidental in this scenario (e.g. at the start of a new page). It should be enough to simply check if the accidental is not equal to nullptr.

[mattmcclinch]

I have modified this PR in light of my previous comment. _accidental == 0 is indeed what I meant, despite what I said about checking if the accidental is not equal to nullptr.