[1.0] Measure number hidden by horizontal frame

MuseScore 1.0. Not reproductible in trunk. r4100.

1. Create score on two systems with default style
2. You should see the measure number on the second system
3. Insert an horizontal frame at the beginning of second system

Expected result : Just the insertion of the frame.
Actual result : the frame is inserted but the measure number is removed.

First report in french : http://musescore.org/fr/node/9846