Fix bug that nghttp2_session_find_stream(session, 0) returned NULL

Previously, nghttp2_session_find_stream(session, 0) returned NULL
despite the fact that documentation said that it should return root
stream.  Now it is corrected, and it returns root stream as
documented.

lib/nghttp2_session.c

@@ -6768,6 +6768,10 @@ int32_t nghttp2_session_get_last_proc_stream_id(nghttp2_session *session) {
 
 nghttp2_stream *nghttp2_session_find_stream(nghttp2_session *session,
                                             int32_t stream_id) {
+  if (stream_id == 0) {
+    return &session->root;
+  }
+
   return nghttp2_session_get_stream_raw(session, stream_id);
 }
 

tests/main.c

@@ -251,6 +251,8 @@ int main(int argc _U_, char *argv[] _U_) {
                    test_nghttp2_session_stream_get_state) ||
       !CU_add_test(pSuite, "session_stream_get_something",
                    test_nghttp2_session_stream_get_something) ||
+      !CU_add_test(pSuite, "session_find_stream",
+                   test_nghttp2_session_find_stream) ||
       !CU_add_test(pSuite, "session_keep_closed_stream",
                    test_nghttp2_session_keep_closed_stream) ||
       !CU_add_test(pSuite, "session_keep_idle_stream",

tests/nghttp2_session_test.c

@@ -7439,6 +7439,34 @@ void test_nghttp2_session_stream_get_something(void) {
   nghttp2_session_del(session);
 }
 
+void test_nghttp2_session_find_stream(void) {
+  nghttp2_session *session;
+  nghttp2_session_callbacks callbacks;
+  nghttp2_stream *stream;
+
+  memset(&callbacks, 0, sizeof(callbacks));
+
+  nghttp2_session_server_new(&session, &callbacks, NULL);
+
+  open_stream(session, 1);
+
+  stream = nghttp2_session_find_stream(session, 1);
+
+  CU_ASSERT(NULL != stream);
+  CU_ASSERT(1 == stream->stream_id);
+
+  stream = nghttp2_session_find_stream(session, 0);
+
+  CU_ASSERT(&session->root == stream);
+  CU_ASSERT(0 == stream->stream_id);
+
+  stream = nghttp2_session_find_stream(session, 2);
+
+  CU_ASSERT(NULL == stream);
+
+  nghttp2_session_del(session);
+}
+
 void test_nghttp2_session_keep_closed_stream(void) {
   nghttp2_session *session;
   nghttp2_session_callbacks callbacks;

tests/nghttp2_session_test.h

@@ -117,6 +117,7 @@ void test_nghttp2_session_stream_attach_item(void);
 void test_nghttp2_session_stream_attach_item_subtree(void);
 void test_nghttp2_session_stream_get_state(void);
 void test_nghttp2_session_stream_get_something(void);
+void test_nghttp2_session_find_stream(void);
 void test_nghttp2_session_keep_closed_stream(void);
 void test_nghttp2_session_keep_idle_stream(void);
 void test_nghttp2_session_detach_idle_stream(void);