Avoid negative values in paintIndentMarker

An e.g., case where this happens:

xPos: -929
line + text: 85 "        connect(provider, &QObject::destroyed, this, [this](QObject *provider) {"
File: "KateTextHintManager.cpp"
x: 4 , w: 9 , xStart:  966 indentWidth: 4 lastIndentColumn:  8