fix #274204: Regression: Ambitus causes a crash just while clicking on it

[mattmcclinch]

See https://musescore.org/en/node/274204.

[Jojo-Schmitz]

Travis is not happy, but not due to the change in this PR, all meets passed

[mattmcclinch]

Jojo-Schmitz wrote:
And/or extend the local array with HEAD_CUSTOM?

There is no corresponding option in the combo box for HEAD_CUSTOM, so it would have to be added to inspector_ambitus.ui as well. But do we even want to allow custom noteheads in Ambitus?

[Jojo-Schmitz]

I'm not sure why we allow different noteheads at all, after all this is for early music. But if we allow them, then I don't see why excluding some.
Then again that would add new strings to translate, so I don't think we want it for 2.3.2.

[mattmcclinch]

As I understand it, custom noteheads are only used for Drumset staves, and the ambitus is only meaningful for pitched instruments.

[Jojo-Schmitz]

We might want to discuss this for master, for 2.3.2 this PR is good as it is IMHO