Don't crash when freezing a struct with a circular reference

The Freeze method needs to check for a `frozen` condition just like all the other types of values, otherwise there can be infinite recursion.

The test that I'm adding here would crash without the fix.

starlarkstruct/struct.go

@@ -99,6 +99,7 @@ func FromStringDict(constructor starlark.Value, d starlark.StringDict) *Struct {
 type Struct struct {
 	constructor starlark.Value
 	entries     entries // sorted by name
+	frozen      bool
 }
 
 // Default is the default constructor for structs.
@@ -172,8 +173,11 @@ func (s *Struct) Hash() (uint32, error) {
 	return x, nil
 }
 func (s *Struct) Freeze() {
-	for _, e := range s.entries {
-		e.value.Freeze()
+	if !s.frozen {
+		s.frozen = true
+		for _, e := range s.entries {
+			e.value.Freeze()
+		}
 	}
 }
 

starlarkstruct/testdata/struct.star

@@ -61,3 +61,12 @@ assert.eq(alice + bob, person(age = 50, city = "NYC", name = "bob"))  # not comm
 assert.fails(lambda : alice + 1, "struct \\+ int")
 assert.eq(http + http, http)
 assert.fails(lambda : http + bob, "different constructors: hostport \\+ person")
+
+# Check that a struct with a circular reference doesn't crash when it gets frozen.
+def struct_with_a_circular_reference():
+    foo = lambda: print(self)
+    self = struct(foo = foo)
+    return self
+
+# A top-level assigment is what causes this value to be frozen at the end of the load.
+toplevel_struct_with_a_circular_reference = struct_with_a_circular_reference()