http: Fix router process crash whilst using proxy

When the client closes the connection before the upstream,
the proxy's error handler was calling cleanup operation like
peer close and request close twice, this fix ensures the cleanup
is performed only once, improving proxy stability.

Closes: #828

src/nxt_h1proto.c

@@ -2869,6 +2869,11 @@ nxt_h1p_peer_body_process(nxt_task_t *task, nxt_http_peer_t *peer,
     } else if (h1p->remainder > 0) {
         length = nxt_buf_chain_length(out);
         h1p->remainder -= length;
+
+        if (h1p->remainder == 0) {
+            nxt_buf_chain_add(&out, nxt_http_buf_last(peer->request));
+            peer->closed = 1;
+        }
     }
 
     peer->body = out;

src/nxt_http_proxy.c

@@ -381,9 +381,10 @@ nxt_http_proxy_error(nxt_task_t *task, void *obj, void *data)
     r = obj;
     peer = r->peer;
 
-    nxt_http_proto[peer->protocol].peer_close(task, peer);
-
-    nxt_mp_release(r->mem_pool);
+    if (!peer->closed) {
+        nxt_http_proto[peer->protocol].peer_close(task, peer);
+        nxt_mp_release(r->mem_pool);
+    }
 
     nxt_http_request_error(&r->task, r, peer->status);
 }