http2: get rid of another strstr()

Follow-up to 1514c446: replace another strstr() call done on a
buffer that might not be zero terminated - with a memchr() call, even if
we know the substring will be found.

Assisted-by: Max Dymond

Detected by OSS-Fuzz
Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8021

Closes #2534