Fix h2 codec state after bad priority header.

Summary:
It's possible for the http2 codec to enter an invalid state after processing a http2 header with invalid priorities.
CVE-2018-6346

Reviewed By: maxgeorg

Differential Revision: D13510025

fbshipit-source-id: 7c4e42daf1cd2b912454d13a66ab8488d1863263