json: fix default struct field initialization with long array (#23355)

vlib/json/tests/json_decode_struct_default_test.v

@@ -0,0 +1,20 @@
+import json
+
+struct Bar {
+	b []int = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+}
+
+struct Foo {
+	Bar
+	a []int = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+}
+
+fn test_main() {
+	str := json.encode(Foo{})
+	assert json.decode(Foo, str)!.str() == 'Foo{
+    Bar: Bar{
+        b: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+    }
+    a: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+}'
+}

vlib/v/gen/c/json.v

@@ -848,8 +848,7 @@ fn (mut g Gen) gen_struct_enc_dec(utyp ast.Type, type_info ast.TypeInfo, styp st
 				if field.has_default_expr {
 					dec.writeln('\t} else {')
 					default_str := g.expr_string_opt(field.typ, field.default_expr)
-					lines := default_str.count('\n')
-					if lines > 1 {
+					if default_str.count(';\n') > 1 {
 						dec.writeln(default_str.all_before_last('\n'))
 						dec.writeln('\t\t${prefix}${op}${c_name(field.name)} = ${default_str.all_after_last('\n')};')
 					} else {