Postpone filter: handled ngx_http_postpone_filter_add() failures.

In particular, if ngx_http_postpone_filter_add() fails in ngx_chain_add_copy(),
the output chain of the postponed request was left in an invalid state.

src/http/ngx_http_postpone_filter_module.c

diff --git a/src/http/ngx_http_postpone_filter_module.c b/src/http/ngx_http_postpone_filter_module.c
index e893b83..55f2698 100644
--- a/src/http/ngx_http_postpone_filter_module.c
+++ b/src/http/ngx_http_postpone_filter_module.c
@@ -63,7 +63,10 @@
     if (r != c->data) {
 
         if (in) {
-            ngx_http_postpone_filter_add(r, in);
+            if (ngx_http_postpone_filter_add(r, in) != NGX_OK) {
+                return NGX_ERROR;
+            }
+
             return NGX_OK;
         }
 
@@ -86,7 +89,9 @@
     }
 
     if (in) {
-        ngx_http_postpone_filter_add(r, in);
+        if (ngx_http_postpone_filter_add(r, in) != NGX_OK) {
+            return NGX_ERROR;
+        }
     }
 
     do {