checker: fix checking uninitialized refs

vlib/v/checker/checker.v

@@ -330,7 +330,7 @@ pub fn (mut c Checker) struct_init(struct_init mut ast.StructInit) table.Type {
 			}
 			// Check uninitialized refs
 			for field in info.fields {
-				if field.name in inited_fields {
+				if field.has_default_expr || field.name in inited_fields {
 					continue
 				}
 				if field.typ.is_ptr() {

vlib/v/checker/tests/reference_field_must_be_initialized.out

@@ -1,7 +1,7 @@
 vlib/v/checker/tests/reference_field_must_be_initialized.v:8:7: error: reference field `Node.next` must be initialized
     6 |
     7 | fn main(){
-    8 |     n := Node{ data: 123 }    
+    8 |     n := Node{ data: 123 }
       |          ~~~~~~~~~~~~~~~~~
     9 |     eprintln('n.data: $n.data')
    10 | }

vlib/v/checker/tests/reference_field_must_be_initialized.vv

@@ -1,10 +1,10 @@
 module main
 struct Node {
     data int
-	next &Node = 0
+	next &Node
 }
 
 fn main(){
-	n := Node{ data: 123 }    
+	n := Node{ data: 123 }
     eprintln('n.data: $n.data')
 }