[core] clear buffer after backend dechunk

(thx flynn)

clear buffer after backend dechunk if not sending chunked to client

x-ref:
  "Memory fragmentation with HTTP/2 enabled"
  https://redmine.lighttpd.net/issues/3084

src/http_chunk.c

@@ -482,6 +482,8 @@ int http_chunk_decode_append_buffer(request_st * const r, buffer * const mem)
         r->resp_send_chunked = 1;
         return rc;
     }
+    else
+        buffer_clear(mem);
 
     return 0;
 }