BUG/MAJOR: mux-h1: Don't decrement .curr_len for unsent data

BUG/MAJOR: mux-h1: Don't decrement .curr_len for unsent data

A regression was introduced by commit 140f1a58 ("BUG/MEDIUM: mux-h1: Fix
splicing by properly detecting end of message"). To detect end of the
outgoing message, when the content-length is announced, we count amount of
data already sent. But only data really sent must be counted.

If the output buffer is full, we can fail to send data (fully or
partially). In this case, we must take care to only count sent
data. Otherwise we may think too much data were sent and an internal error
may be erroneously reported.

This patch should fix issues #1510 and #1511. It must be backported as far
as 2.4.