Reject zero length handshake records.

Summary:
Zero length (all padding) handshake are forbidden by RFC. Allowing
these was a regression in D13754697 (2c6f78a).

This is a partial fix for CVE-2019-11924

Reviewed By: xybu

Differential Revision: D16285100

fbshipit-source-id: 05a19d31ad74601ce89156a0e59517aaad8dd928