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

@@ -781,7 +781,7 @@ fn eval_const_expr(
 ) -> Option<u16> {
     match expression {
         Expression::NumberLiteral(v, Unit::None) => {
-            if *v < 0. || *v > u16::MAX as f64 || !v.trunc().approx_eq(v) {
+            if *v < 1. || *v > u16::MAX as f64 || !v.trunc().approx_eq(v) {
                 diag.push_error(format!("'{}' must be a positive integer", name), span);
                 None
             } else {

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; }