GUI2: prevented Scrollbar (and by extension Slider) step size from be…

…ing set to 0

This can potentially cause division-by-0 crashes.

src/gui/widgets/scrollbar.hpp

@@ -162,6 +162,11 @@ class scrollbar_base : public styled_widget
 
 	void set_step_size(const unsigned step_size)
 	{
+		// Step size can't be 0!
+		if(step_size == 0) {
+			throw std::invalid_argument("GUI2: scrollbar step size cannot be 0");
+		}
+
 		step_size_ = step_size;
 		recalculate();
 	}

src/gui/widgets/slider.cpp

@@ -468,7 +468,7 @@ builder_slider::builder_slider(const config& cfg)
 	, best_slider_length_(cfg["best_slider_length"])
 	, minimum_value_(cfg["minimum_value"])
 	, maximum_value_(cfg["maximum_value"])
-	, step_size_(cfg["step_size"])
+	, step_size_(cfg["step_size"].to_unsigned(1))
 	, value_(cfg["value"])
 	, minimum_value_label_(cfg["minimum_value_label"].t_str())
 	, maximum_value_label_(cfg["maximum_value_label"].t_str())