Fix runtime panic with colpan or rowspan set to zero

Make it a compiler error instead:
Fix panic:
```
panicked at internal/core/layout.rs:459:33:
index out of bounds: the len is 2 but the index is 2
```

ChangeLog: Transformed runtime panic with `colspan:0` or `rowspan:0` into a compile error

internal/compiler/passes/lower_layout.rs

@@ -270,20 +270,18 @@ impl GridLayout {
         layout_cache_prop_v: &NamedReference,
         diag: &mut BuildDiagnostics,
     ) {
-        let mut get_const_value = |name: &str| {
-            item_element
-                .borrow_mut()
-                .bindings
-                .remove(name)
-                .and_then(|e| eval_const_expr(&e.borrow().expression, name, &*e.borrow(), diag))
+        let mut get_const_value = |name: &str, min| {
+            item_element.borrow_mut().bindings.remove(name).and_then(|e| {
+                eval_const_expr(&e.borrow().expression, name, min, &*e.borrow(), diag)
+            })
         };
-        let colspan = get_const_value("colspan").unwrap_or(1);
-        let rowspan = get_const_value("rowspan").unwrap_or(1);
-        if let Some(r) = get_const_value("row") {
+        let colspan = get_const_value("colspan", 1.).unwrap_or(1);
+        let rowspan = get_const_value("rowspan", 1.).unwrap_or(1);
+        if let Some(r) = get_const_value("row", 0.) {
             *row = r;
             *col = 0;
         }
-        if let Some(c) = get_const_value("col") {
+        if let Some(c) = get_const_value("col", 0.) {
             *col = c;
         }
 
@@ -776,19 +774,20 @@ fn set_prop_from_cache(
 fn eval_const_expr(
     expression: &Expression,
     name: &str,
+    min: f64,
     span: &dyn crate::diagnostics::Spanned,
     diag: &mut BuildDiagnostics,
 ) -> Option<u16> {
     match expression {
         Expression::NumberLiteral(v, Unit::None) => {
-            if *v < 0. || *v > u16::MAX as f64 || !v.trunc().approx_eq(v) {
+            if *v < min || *v > u16::MAX as f64 || !v.trunc().approx_eq(v) {
                 diag.push_error(format!("'{}' must be a positive integer", name), span);
                 None
             } else {
                 Some(*v as u16)
             }
         }
-        Expression::Cast { from, .. } => eval_const_expr(from, name, span, diag),
+        Expression::Cast { from, .. } => eval_const_expr(from, name, min, span, diag),
         _ => {
             diag.push_error(format!("'{}' must be an integer literal", name), span);
             None

internal/compiler/tests/syntax/basic/layout2.slint

@@ -42,6 +42,14 @@ export X := Rectangle {
             Rectangle { row: 3; }
 //                           ^error{row used outside of a GridLayout}
         }
+
+        Rectangle {
+            // This would cause panic at runtime
+            rowspan: 0;
+            //       ^error{'rowspan' must be a positive integer}
+            colspan: 0;
+            //       ^error{'colspan' must be a positive integer}
+        }
     }
 
     Text { colspan: 3; }