http2: shrink memory to match read data

Perform a shrinking `Realloc()` so that less data is
used for HTTP2 reads.

PR-URL: https://github.com/nodejs/node/pull/26201


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>