Fix measures widths produces shortened line if horizontal frame present

Windows 7, GIT commit: fa00a2a

1) create new score, one instrument, 8 measures
2) insert horizontal frame in front of first measure
3) add linebreak after measure 4
4) Style / General / System, turn on Fix measure widths

Result: first line is shortened

Note: same thing happened in 1.3; this isn't a regression

See also #5786: "Fix Measure Width" causes notes to line up incorrectly on the first stave.