fix: initialize buffer in cipher_openssl.c

[ajewellamz]

Issue #, if available:
#749

Description of changes:
Initialize the uninitialized buffer. It will cost a few instructions, but it's not in a tight loop.

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[alex-chew]

I'm going ahead and merging this despite the two failing checks. The csdk-ubuntu-latest-x64-awslc check will fail until we merge #750, which in turn depends (functionally) on this PR. And it's safe to ignore the Semantic PR check since I'll fix up the squash commit message.