src/java.base/share/native/libzip/zip_util.c

@@ -1204,11 +1204,11 @@
         if (!addSlash) {
             break;
         }
 
         /* Slash is already there? */
-        if (name[ulen-1] == '/') {
+        if (ulen > 0 && name[ulen - 1] == '/') {
             break;
         }
 
         /* Add slash and try once more */
         name[ulen++] = '/';