[1.0] Measure num. every n shows number (every n) + 1 (with patch)

Context: ver. 1.0 rev. 4156

Issue: If the measure number is set to appear every n measures (menu "Style | Edit general style | Numbers", Interval), the measure number is actually shown (every n measures) +1.

Example: if measure number is set every 5 measures, measures with number are 6, 11, 16, ...

Expected result: measure number is shown for measure 5, 10, 15, ... as it is customary.

The small attached patch corrects the file measure.cpp to have the correct numbering.