Fix HelloVerify patch not applied and release 2.5.1

This is merge request for bug fix release that fixes remove helloverify countermeasure was not effective in last release.

The root cause was that a line of edit was lost during the backporting process(https://github.com/xiaokangwang/dtls/commit/16e5cc8ce01c0262e4d945b7fe90eba4d7d58ce5)

X~X maybe we will need to repeat the test before actually merging in the future to reduce this kind of mistake.