Check for control flow event before let binding (#1912)

Fixes #1906 (trying to destructure the result of `continue`).

crates/typst/src/eval/mod.rs

@@ -1470,6 +1470,9 @@ impl Eval for ast::LetBinding<'_> {
             Some(expr) => expr.eval(vm)?,
             None => Value::None,
         };
+        if vm.flow.is_some() {
+            return Ok(Value::None);
+        }
 
         match self.kind() {
             ast::LetBindingKind::Normal(pattern) => define_pattern(vm, pattern, value),

tests/typ/compiler/break-continue.typ

@@ -145,3 +145,18 @@
     for _ in range(3) [B]
   )
 }
+
+---
+// Ref: true
+// Test continue while destructuring.
+// Should output "one = I \ two = II \ one = I".
+#for num in (1, 2, 3, 1) {
+  let (word, roman) = if num == 1 {
+    ("one", "I")
+  } else if num == 2 {
+    ("two", "II")
+  } else {
+    continue
+  }
+  [#word = #roman \ ]
+}