fixed backescape the first word in buffer

Author: Pavel Vorobyov

runebuf.go

@@ -331,8 +331,8 @@ func (r *RuneBuffer) BackEscapeWord() {
 		if r.idx == 0 {
 			return
 		}
-		for i := r.idx - 1; i > 0; i-- {
-			if !IsWordBreak(r.buf[i]) && IsWordBreak(r.buf[i-1]) {
+		for i := r.idx - 1; i >= 0; i-- {
+			if !IsWordBreak(r.buf[i]) && (i == 0 || IsWordBreak(r.buf[i-1])) {
 				r.pushKill(r.buf[i:r.idx])
 				r.buf = append(r.buf[:i], r.buf[r.idx:]...)
 				r.idx = i