fix: PA pattern calibration crash with first layer line width 0 (#14447)

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

tests/libslic3r/CMakeLists.txt

@@ -8,6 +8,7 @@ add_executable(${_TEST_NAME}_tests
     test_arachne_walls.cpp
     test_arrange.cpp
     test_bambu_networking.cpp
+    test_calib.cpp
     test_clipper_offset.cpp
     test_clipper_utils.cpp
     test_config.cpp