shill: Fix assertion in HTTPRequest::WriteToServer.

BUG=chromium-os:35614
TEST=Build and run unit tests.

Change-Id: I38f8384e779f2ef50c8e54a07ba3441766c6e9c7
Reviewed-on: https://gerrit.chromium.org/gerrit/36321
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
diff --git a/http_request.cc b/http_request.cc
index 98d9020..d924ccd 100644
--- a/http_request.cc
+++ b/http_request.cc
@@ -249,7 +249,7 @@
   CHECK_EQ(server_socket_, fd);
   int ret = sockets_->Send(fd, request_data_.GetConstData(),
                            request_data_.GetLength(), 0);
-  CHECK(static_cast<size_t>(ret) <= request_data_.GetLength());
+  CHECK(ret < 0 || static_cast<size_t>(ret) <= request_data_.GetLength());
 
   SLOG(HTTP, 3) << "In " << __func__ << " wrote " << ret << " of "
                 << request_data_.GetLength();