fully initialize LZ4F_decompressOptions_t
fully initialize LZ4F_decompressOptions_t

Summary:
Reported by MSAN:
{P535152745}

Depending on LZ4 version, failure to fully initialize `LZ4F_decompressOptions_t` may lead to unpredictable work being done (in this case check sum verification).

Reviewed By: felixhandte

Differential Revision: D40041673

fbshipit-source-id: b46f6e5f1d6c09f6fd32d0464909125f93d8981b

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly