Fix #25165 - crash when changing measure with whole measure rest

Fix #25165 - crash when changing the actual duration of measure with single whole-measure rest to or from irregular time.

When the actual duration of a measure is changed and either the starting or ending duration cannot be represented with a simple value (for instance 5/4) and the measure contains a single whole measure rest, a division by 0 results. See http://musescore.org/en/node/25165 for examples, discussion and analysis.

Fixed.

If the new measure value is different from the current time sig, rest(s) to represent the actual measure values are used instead of a single whole measure rest.