DEBUG: cache: Update underlying buffer when loading HTX message in cache applet

In the I/O handler of the cache applet, we must update the underlying buffer
when the HTX message is loaded, using htx_from_buf() function instead of
htxbuf(). It is important because the applet will update the message by
adding new HTX blocks. This way, the state of the underlying buffer remains
consistant with the state of the HTX message.

It is especially important if HAProxy is compiled with "DEBUG_STRICT=2"
mode. Without this patch, channel_add_input() call crashed if the channel
was empty at the begining of the I/O handler.

Note that it is more a build/debug issue than a bug. But this patch may
prevent future bugs. For now it is safe because htx_to_buf() function is
systematically called, updating accordingly the underlying buffer.

This patch may be backported as far as 2.0.

src/cache.c

@@ -1437,7 +1437,7 @@ static void http_cache_io_handler(struct appctx *appctx)
 	unsigned int len;
 	size_t ret, total = 0;
 
-	res_htx = htxbuf(&res->buf);
+	res_htx = htx_from_buf(&res->buf);
 	total = res_htx->data;
 
 	if (unlikely(si->state == SI_ST_DIS || si->state == SI_ST_CLO))