version 2.9.4 git: prevent o-length segments from tripping multiple assertions when viewing layers in vase mode

[themanyone]

Fixes #13739

Optimized if statement so functional operation appears on right-hand side of the ||
That way, faster == checks are performed first and function doesn't operate on 0-length lines.

That prevented some assertions but CoolingBuffer was still trying to process some empty gcode, which is worked-around by skipping empty lines.

+        if (line.length() == 0)
+            continue; // Empty line.
...

Added a bounds check to DoubleSliderForLayers.cpp so operating the slider doesn't crash plater on bottom layer.
Finally, replaced two assertions in Polyline.cpp with if statements to prevent crash when trying to rotate the view.