test-stringlist: avoid buffer underrun when sorting nothing

Check if the strbuf containing data to sort is empty before attempting
to trim a trailing newline character.
Signed-off-by: NRene Scharfe <l.s.r@web.de>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/helper/test-string-list.c

@@ -108,7 +108,7 @@ int cmd_main(int argc, const char **argv)
 		 * Split by newline, but don't create a string_list item
 		 * for the empty string after the last separator.
 		 */
-		if (sb.buf[sb.len - 1] == '\n')
+		if (sb.len && sb.buf[sb.len - 1] == '\n')
 			strbuf_setlen(&sb, sb.len - 1);
 		string_list_split_in_place(&list, sb.buf, '\n', -1);