CalibPressureAdvancePattern::line_width_first_layer() returned the raw
initial_layer_line_width, so a value of 0 (which means "use the default")
fed 0 into the Flow spacing math and threw FlowErrorNegativeSpacing,
crashing the whole app when slicing a PA pattern calibration.
Mirror the guard already present in the sibling line_width(): when the
configured width is non-positive, fall back to auto_extrusion_width.
Add a libslic3r regression test covering the width resolution.
Fixes#13188
f13f67959c