BUG/MINOR: mux-h1: always make sure h1s->sd exists in h1_dump_h1s_info()

BUG/MINOR: mux-h1: always make sure h1s->sd exists in h1_dump_h1s_info()

This function may be called from a signal handler during a warning,
a panic or a show thread. We need to be more cautious about what may
or may not be dereferenced since an h1s is not necessarily fully
initialized. Loops of "show threads" sometimes manage to crash when
dereferencing a null h1s->sd, so let's guard it and add a comment
remining about the unusual call place.

This can be backported to the relevant versions.