parser: fix tmp is already started issue

vlib/compiler/cgen.v

@@ -25,6 +25,7 @@ mut:
 	consts_init  []string
 	pass         Pass
 	nogen           bool
+	prev_tmps    []string
 	tmp_line        string
 	cur_line        string
 	prev_line       string
@@ -105,9 +106,7 @@ fn (g mut CGen) save() {
 
 fn (g mut CGen) start_tmp() {
 	if g.is_tmp {
-		println(g.tmp_line)
-		println('start_tmp() already started. cur_line="$g.cur_line"')
-		exit(1)
+		g.prev_tmps << g.tmp_line
 	}
 	// kg.tmp_lines_pos++
 	g.tmp_line = ''
@@ -115,9 +114,14 @@ fn (g mut CGen) start_tmp() {
 }
 
 fn (g mut CGen) end_tmp() string {
-	g.is_tmp = false
 	res := g.tmp_line
+	if g.prev_tmps.len > 0 {
+		g.tmp_line = g.prev_tmps.last()
+		g.prev_tmps = g.prev_tmps[0..g.prev_tmps.len-1]
+	} else {
 		g.tmp_line = ''
+		g.is_tmp = false
+	}
 	return res
 }