BUILD: http-htx: Silent build error about a possible NULL start-line

In http_replace_req_uri(), if the URI was successfully replaced, it means
the start-line exists. However, the compiler reports an error about a
possible NULL pointer dereference:

src/http_htx.c: In function ‘http_replace_req_uri’:
src/http_htx.c:392:19: error: potential null pointer dereference [-Werror=null-dereference]
  392 |         sl->flags &= ~HTX_SL_F_NORMALIZED_URI;

So, ALREADY_CHECKED() macro is used to silent the build error. This patch
must be backported with 84cdbe478a ("BUG/MINOR: http-htx: Don't consider an
URI as normalized after a set-uri action").
diff --git a/src/http_htx.c b/src/http_htx.c
index 3dfbd01..7cfaca4 100644
--- a/src/http_htx.c
+++ b/src/http_htx.c
@@ -389,6 +389,8 @@
 		goto fail;
 
 	sl = http_get_stline(htx);
+	ALREADY_CHECKED(sl); /* the stline exists because http_replace_stline() succeded */
+
 	sl->flags &= ~HTX_SL_F_NORMALIZED_URI;
 	if (!http_update_host(htx, sl, uri))
 		goto fail;