Fix compile error with gcc and --enable-werror

tests/nghttp2_session_test.c

@@ -3487,14 +3487,11 @@ void test_nghttp2_session_on_altsvc_received(void) {
   nghttp2_session_callbacks callbacks;
   my_user_data ud;
   nghttp2_frame frame;
-  nghttp2_mem *mem;
   nghttp2_option *option;
   uint8_t origin[] = "nghttp2.org";
   uint8_t field_value[] = "h2=\":443\"";
   int rv;
 
-  mem = nghttp2_mem_default();
-
   memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
   callbacks.on_frame_recv_callback = on_frame_recv_callback;
 
@@ -5445,7 +5442,6 @@ void test_nghttp2_submit_extension(void) {
 void test_nghttp2_submit_altsvc(void) {
   nghttp2_session *session;
   nghttp2_session_callbacks callbacks;
-  nghttp2_mem *mem;
   my_user_data ud;
   int rv;
   ssize_t len;
@@ -5453,8 +5449,6 @@ void test_nghttp2_submit_altsvc(void) {
   nghttp2_frame_hd hd;
   size_t origin_len;
 
-  mem = nghttp2_mem_default();
-
   memset(&callbacks, 0, sizeof(nghttp2_session_callbacks));
 
   nghttp2_session_server_new(&session, &callbacks, &ud);