[trunk] Changing beam inclination in note group with clef change inside corrupts it.

1) Create a piano score from template.
2) Write 4 16th-notes one after another.
3) Drag-drop a different clef on the 2nd note (or 3rd or 4th ).
4) Make sure the stems are pointing upwards (clicking 'x').
5) Double-click the beam and move the left square upwards or downwards (ie. change the inclination).

expected result: you can change the inclination of the beam. In most cases this is done to avoid clashes with the clef.
actual result: you get a mess.

That doesn't happen if the stems are pointing downwards.

Thanks.

~Guifré