Don’t check last character of empty strings.

Fixes #511

Author: dillof

THANKS

@@ -67,6 +67,7 @@ Gerard ODonnell
 Giovanni
 gk7huki <gk7huki@gmail.com>
 Hanno Böck <hanno@hboeck.de>
+Harrison Green
 HeeMyung
 Heiko Becker
 Heiko Hund <heiko@ist.eigentlich.net>

lib/zip_file_rename.c

@@ -55,8 +55,8 @@ zip_file_rename(zip_t *za, zip_uint64_t idx, const char *name, zip_flags_t flags
     if ((old_name = zip_get_name(za, idx, 0)) == NULL)
         return -1;
 
-    new_is_dir = (name != NULL && name[strlen(name) - 1] == '/');
-    old_is_dir = (old_name[strlen(old_name) - 1] == '/');
+    new_is_dir = (name != NULL && name[0] != '\0' && name[strlen(name) - 1] == '/');
+    old_is_dir = (old_name[0] != '\0' && old_name[strlen(old_name) - 1] == '/');
 
     if (new_is_dir != old_is_dir) {
         zip_error_set(&za->error, ZIP_ER_INVAL, 0);