HTTP/2: cleaned up state while closing stream.

Without this the state might keep pointing to already closed stream.

src/http/v2/ngx_http_v2.c

diff --git a/src/http/v2/ngx_http_v2.c b/src/http/v2/ngx_http_v2.c
index 4e378c9..0447534 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -3661,6 +3661,10 @@
         }
     }
 
+    if (h2c->state.stream == stream) {
+        h2c->state.stream = NULL;
+    }
+
     node->stream = NULL;
 
     ngx_queue_insert_tail(&h2c->closed, &node->reuse);