Fix wrong condition in weaved

I thought I have fixed this earlier... Apparently not...

Change-Id: Idf01e6e45c1f6f019fe22fdaf22b6fd2045141ff
diff --git a/buffet/webserv_client.cc b/buffet/webserv_client.cc
index 31122ab..f165849 100644
--- a/buffet/webserv_client.cc
+++ b/buffet/webserv_client.cc
@@ -34,7 +34,7 @@
   // TODO(avakulenko): Remove this method and rewrite all call sites in libweave
   // to use GetDataStream() instead.
   const std::vector<uint8_t>& GetData() const override {
-    if (!request_data_)
+    if (request_data_)
       return *request_data_;
 
     request_data_.reset(new std::vector<uint8_t>);