checker: fix nested struct reference type field initialized check. (fix: #15741) (#15752)

vlib/net/html/dom.v

@@ -9,7 +9,7 @@ import os
 // https://www.w3.org/TR/WD-DOM/introduction.html
 pub struct DocumentObjectModel {
 mut:
-	root           &Tag
+	root           &Tag = unsafe { nil }
 	constructed    bool
 	btree          BTree
 	all_tags       []&Tag

vlib/net/html/parser.v

@@ -5,7 +5,7 @@ import strings
 
 struct LexicalAttributes {
 mut:
-	current_tag      &Tag
+	current_tag      &Tag = unsafe { nil }
 	open_tag         bool
 	open_code        bool
 	open_string      int

vlib/net/html/tag.v

@@ -16,7 +16,7 @@ pub mut:
 	children           []&Tag
 	attributes         map[string]string // attributes will be like map[name]value
 	last_attribute     string
-	parent             &Tag = unsafe { 0 }
+	parent             &Tag = unsafe { nil }
 	position_in_parent int
 	closed             bool
 	close_type         CloseTagType = .in_name