[MU3] Avoid deprecated assignment past the end of QString

[orivej]

https://doc.qt.io/qt-5/qstring.html#operator-5b-5d says this about
QCharRef QString::operator[](int position):

"Before Qt 5.14 it was possible to use this operator to access a
character at an out-of-bounds position in the string, and then assign
to such a position, causing the string to be automatically
resized. These behaviors are deprecated, and will be changed in a
future version of Qt."

Corresponding v4 PR: #7125

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[Jojo-Schmitz]

this is changing the old 3.x code style and its "banner style indent". Not a big deal though, more a nitpick...

[orivej]

Oops, I've brought the old style back.