Fix #4188 - Check for failed std::find

[Veratil]

I noticed this crashed on stable-1.2 and then in master.

Crash happens when trying to erase a note that doesn't exist in m_markedSemiTones when selecting "Mark/unmark all corresponding octave semitones", so the solution is just to check for a failed std::find.

[Reflexe]

@PhysSong I'm not sure, Do we have to target it for stable and sync it later to master? Or just do it manually now?

[PhysSong]

@Reflexe Since #4188 is in the 1.2.1 milestone, the commit should somehow go to stable-1.2. The specific method doesn't matter too much, but I think it'd be better to merge into stable-1.2 first.
This should be tested before we merge this, of course.

[Reflexe]

Since we've changed qFind to std::find between stable-2 and master, I;ve created another pr for stable: #5184