BUG/MINOR: Don't report early srv aborts on request forwarding in DONE state

L7-retries may be ignored if server aborts are detected during the request
forwarding, when the request is already in DONE state.

When a request was fully processed (so in HTTP_MSG_DONE state) and is
waiting for be forwarded to the server, there is a test to detect server
aborts, to be able to report the error. However, this test must be skipped
if the response was not received yet, to let the reponse analyszers handle
the abort. It is important to properly handle the retries. This test must
only be performed if the response analysis was finished. It means the
response must be at least in HTTP_MSG_BODY state.

This patch should be backported as far as 2.8.

src/http_ana.c

@@ -1012,7 +1012,7 @@ int http_request_forward_body(struct stream *s, struct channel *req, int an_bit)
 		 * response. Otherwise, let a change to forward the response
 		 * first.
 		 */
-		if (htx_is_empty(htxbuf(&s->res.buf))) {
+		if (txn->rsp.msg_state >= HTTP_MSG_BODY && htx_is_empty(htxbuf(&s->res.buf))) {
 			COUNT_IF(1, "Server abort during request forwarding");
 			goto return_srv_abort;
 		}