Fix #317747: Measure number appears after edit in first measure after section break with intervening frame

[mattmcclinch]

Resolves: https://musescore.org/en/node/317747.

This was just a simple case of forgetting to update mb with the result of mb->findPotentialSectionBreak().

[mattmcclinch]

This is handled in #7569.

[MarcSabatella]

Hah, I see you submitted yours first, nice job! Really, I'm not sure the second change I made is strictly needed, certainly it isn't just for this exact case. But it could be in other cases, and certainly shouldn't hurt.

[mattmcclinch]

I included a nullptr check in mine. You might consider doing the same in yours in both places, since findPotentialSectionBreak() can potentially return nullptr.