BUG/MEDIUM: mux-h1: Be sure xprt support splicing to use it during fast-forward

BUG/MEDIUM: mux-h1: Be sure xprt support splicing to use it during fast-forward

The commit d6d4abdc3 ("BUILD: mux-h1: Fix build without kernel splicing
support") introduced a regression. The kernel support for the underlying
XPRT is no longer checked. So it is possible to enable the splicing for SSL
connection. This of course leads to a segfault.

This patch restore the test on the xprt rcv_pipe/snd_pipe functions.

This patch should fix a crash reported by Tristan in #2095
(#issuecomment-1788949014). No backport needed.