Do not crash on invalid lines

ftp.go

@@ -403,10 +403,12 @@ func parseDirListLine(line string) (*Entry, error) {
 
 	// Try various time formats that DIR might use, and stop when one works.
 	for _, format := range dirTimeFormats {
-		e.Time, err = time.Parse(format, line[:len(format)])
-		if err == nil {
-			line = line[len(format):]
-			break
+		if len(line) > len(format) {
+			e.Time, err = time.Parse(format, line[:len(format)])
+			if err == nil {
+				line = line[len(format):]
+				break
+			}
 		}
 	}
 	if err != nil {