HTTP/2: lingering close changed to handle NGX_AGAIN.

This part somehow slipped away from c5840ca2063d.

While it is not expected to be needed in case of lingering close,
it is good to keep it for correctness (see 2b5528023f6b).

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 77b60d4..7c1ba59 100644
--- a/src/http/v2/ngx_http_v2.c
+++ b/src/http/v2/ngx_http_v2.c
@@ -807,6 +807,10 @@
 
         ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0, "lingering read: %z", n);
 
+        if (n == NGX_AGAIN) {
+            break;
+        }
+
         if (n == NGX_ERROR || n == 0) {
             ngx_http_close_connection(c);
             return;