CLEANUP: mux-h2: fix minor output debugging format issues

CLEANUP: mux-h2: fix minor output debugging format issues

In h2_dump_h2s_info(), the tl.calls was being printed as signed instead
of unsigned, which is not correct but harmless (only used with "show
fd"). In the same function, we don't check if h2s->sd is valid while
dereferencing it. In practise it is valid since "show fd" is run under
thread isolation, but it's far from being obvious, and if conditions
would later change, we don't know it could be printed between h2s_new()
and h2s_frt_stream_new(). Finally in h2s_make_data() a wrong set of
H2_EV_RX_* flags were used instead of H2_EV_TX_* to emit traces.