8079841: Buffer underflow with empty zip entry names

Check for zero-length entry name before accessing "last" byte

Reviewed-by: sherman, martin

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

@@ -1206,7 +1206,7 @@ ZIP_GetEntry2(jzfile *zip, char *name, jint ulen, jboolean addSlash)
         }
 
         /* Slash is already there? */
-        if (name[ulen-1] == '/') {
+        if (ulen > 0 && name[ulen - 1] == '/') {
             break;
         }