BUG/MINOR: mux-h1: Process input even if the input buffer is empty

It is required, at least, to add the EOM block and finish the message when the
TCP splicing was used to send all data. Otherwise, there is no way to finish the
parsing.

This patch must be backported to 1.9.

src/mux_h1.c

@@ -1319,8 +1319,6 @@ static size_t h1_process_input(struct h1c *h1c, struct buffer *buf, int flags)
 
 	data = htx->data;
 	count = b_data(&h1c->ibuf);
-	if (!count)
-		goto end;
 	rsv = ((flags & CO_RFL_KEEP_RSV) ? global.tune.maxrewrite : 0);
 
 	if (htx_is_empty(htx))