Update SDK with secure tunneling null websocket fix

[marcoemorais-aws]

Motivation

• Fix segmentation fault when closing secure tunnel with null websocket

Modifications

Change summary

• Bump SDK to pick up secure tunnel fix in aws-c-iot b50828c

Revision diff summary

If there is more than one revision, please explain what has been changed since the last revision.

Testing

• Users who enable secure tunneling and shutdown the device client before the websocket connection is established would observe a segmentation fault. That behavior is fixed as part of this commit.

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