BUG/MINOR: mux-h1: Disable splicing only if input data was processed

BUG/MINOR: mux-h1: Disable splicing only if input data was processed

In h1_rcv_buf(), the splicing is systematically disabled if it was previously
enabled. When it happens, if the splicing is enabled it means the channel's
buffer was empty before calling h1_rcv_buf(). Thus, the only reason to disable
the splicing at this step is when some input data have just been processed.

This patch may be backported to 2.1 and 2.0.

(cherry picked from commit 7b7016bf6e69f61074684e9145e1a6784b6a9d03)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>