BUG/MEDIUM: mux-h2: use the correct offset for the HTX start line

BUG/MEDIUM: mux-h2: use the correct offset for the HTX start line

Due to a thinko, I used sl_off as the start line index number but it's
not it, it's its offset. The first index is obtained using htx_get_head(),
and the start line is obtained using htx_get_sline(). This caused crashes
to happen when forwarding HTX traffic via the H2 mux once the HTX buffer
started to wrap.

No backport is needed.