cgen: fix parallel cgen for json encoding of struct fields that have default values

vlib/v/gen/c/cgen.v

@@ -283,6 +283,7 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 	global_g.init()
 	global_g.timers.show('cgen init')
 	global_g.tests_inited = false
+	global_g.file = files.last()
 	if !pref.no_parallel {
 		mut pp := pool.new_pool_processor(callback: cgen_process_one_file_cb)
 		pp.set_shared_context(global_g) // TODO: make global_g shared
@@ -2886,7 +2887,7 @@ fn (mut g Gen) expr(node_ ast.Expr) {
 					g.writeln('($shared_styp*)__dup${shared_styp}(&($shared_styp){.mtx = {0}, .val =')
 				}
 			}
-			last_stmt_pos := g.stmt_path_pos.last()
+			last_stmt_pos := if g.stmt_path_pos.len > 0 { g.stmt_path_pos.last() } else { 0 }
 			g.call_expr(node)
 			// if g.fileis('1.strings') {
 			// println('before:' + node.autofree_pregen)

vlib/v/tests/json_with_struct_having_fields_with_default_values_test.v

@@ -0,0 +1,27 @@
+import json
+
+struct Window {
+pub mut:
+	width  f64
+	height f64
+}
+
+fn make_default_window_settings() Window {
+	return Window{
+		width: 1280
+		height: 720
+	}
+}
+
+struct Settings {
+pub mut:
+	window Window = make_default_window_settings()
+}
+
+fn test_encoding_works() {
+	mut settings := Settings{}
+	dump(settings)
+	encoded := json.encode(settings)
+	println(encoded)
+	assert encoded == '{"window":{"width":1280,"height":720}}'
+}