cgen: fix option struct default value init with -cstrict (spotted in #22783) (#22802)

vlib/v/gen/c/struct.v

@@ -107,7 +107,7 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 			...node
 			typ: node.typ.clear_option_and_result()
 		})
-		g.writeln('}, &${tmp_var}, sizeof(${base_styp}));')
+		g.writeln('}, (${option_name}*)&${tmp_var}, sizeof(${base_styp}));')
 		g.empty_line = false
 		g.write2(s, tmp_var)
 		return

vlib/v/tests/options/option_struct_default_test.v

@@ -0,0 +1,16 @@
+struct Element {
+	a int
+}
+
+@[noinit]
+struct RawElement {
+mut:
+	// tag Tag
+	content       []u8
+	default_value ?Element
+}
+
+fn test_main() {
+	a := ?RawElement{}
+	dump(a)
+}