BUG/MINOR: mux-h2: harden h2_dump_h2s_info() against potentially null h2s->sd

BUG/MINOR: mux-h2: harden h2_dump_h2s_info() against potentially null h2s->sd

Not really sure if this can happen but since the code already takes care
of testing the sd, better make sure everywhere it's dereferenced it's
tested. And h2s_sc() does dereference h2s->sd which is later tested in
that function.

Under normal circumstances it's fine (when called from the stream via
h2_show_fd()) but it's less certain when dumping FDs via h2_show_fd()
which is called by the CLI's "show fd" debugging command. Let's just
guard all h2s_sc() under an sd test then.

Thanks to Ilya for reporting this Coverity finding in GH issue #3475.

This can be backported wherever it applies.