Fix handling of empty ANSI color sequence

Fix #3320

src/ansi.go

@@ -351,9 +351,11 @@ func interpretCode(ansiCode string, prevState *ansiState) ansiState {
 	ptr := &state.fg
 
 	var delimiter byte = 0
+	count := 0
 	for len(ansiCode) != 0 {
 		var num int
 		if num, delimiter, ansiCode = parseAnsiCode(ansiCode, delimiter); num != -1 {
+			count++
 			switch state256 {
 			case 0:
 				switch num {
@@ -435,6 +437,13 @@ func interpretCode(ansiCode string, prevState *ansiState) ansiState {
 		}
 	}
 
+	// Empty sequence: reset
+	if count == 0 {
+		state.fg = -1
+		state.bg = -1
+		state.attr = 0
+	}
+
 	if state256 > 0 {
 		*ptr = -1
 	}