http2: convert an assert to run-time check

Fuzzing has proven we can reach code in on_frame_recv with status_code
not having been set, so let's detect that in run-time (instead of with
assert) and error error accordingly.

(This should no longer happen with the latest nghttp2)

Detected by OSS-Fuzz
Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7903
Closes #2514