BUG/MINOR: mux-h1: Only skip invalid C-L headers on output

BUG/MINOR: mux-h1: Only skip invalid C-L headers on output

When an HTTP request with an empty body is received, the flag HTX_SL_F_BODYLESS
is set on the HTX start-line block. It is true if the header content-length is
explicitly set to 0 or if it is omitted for a non chunked request.

On the server side, when the request is reformatted, because HTX_SL_F_BODYLESS
is set, the flag H1_MF_CLEN is added on the request parser. It is done to not
add an header transfer-encoding on bodyless requests. But if an header
content-length is explicitly set to 0, when it is parsed, because H1_MF_CLEN is
set, the function h1_parse_cont_len_header() returns 0, meaning the header can
be dropped. So in such case, a request without any header content-length is sent
to the server.

Some servers seems to reject empty POST requests with an error 411 when there is
no header content-length. So to fix this issue, on the output side, only headers
with an invalid content length are skipped, ie only when the function
h1_parse_cont_len_header() returns a negative value.

This patch must be backported to 1.9.