fix #116556: Crash by opening the Instruments dialog box after parts …

[heuchi]

…deletion and undo

Ms::deleteExcerpt() did not make sure it really uses a staff of the master score to unlink staves of the excerpt. The loop using calls to Staff::primaryStaff() depends on the order of elements in _linkedStaves, since Staff::primaryStaff() will always return true if only two staves of two different scores are in the list.

With the given order of steps in #116556 this could cause the final call to

oscore->undo(new UnlinkStaff(staff, s));

having (staff == s) both pointing to the same staff in the excerpt. The pointer to _linkedStaves in the master score was therefore never set to 0, while the LinkedStaves object itself was deleted.

Opening the Instruments Dialog window made use of this invalid _linkedStaves pointer and crashed.