prevent from reading null (#3115)

* prevent from reading nullptr

src/core/storage/sframe_interface/unity_sframe.cpp

@@ -1856,6 +1856,8 @@ void unity_sframe::delete_on_close() {
 }
 
 std::shared_ptr<planner_node> unity_sframe::get_planner_node() {
+  ASSERT_MSG(m_planner_node != nullptr,
+             "Unintialized SFrame planner node cannot be used for read");
   return m_planner_node;
 }
 

src/python/turicreate/test/test_io_s3.py

@@ -129,6 +129,20 @@ def test_s3_sframe_upload(self, folder):
         # we can trust the upload becuase if the upload fails,
         # s3 will respond with 5xx
 
+    @pytest.mark.parametrize(
+        "url",
+        ["s3://gui/dummy", "./willy-nily/blah", "https://foo.com", "http://hao.com"],
+    )
+    def test_s3_sframe_load_from_wrong_path(self, url):
+        # s3 only writes when it receives all parts
+        # it's sort of atmoic write on file level.
+        if six.PY2:
+            with pytest.raises(IOError):
+                SFrame(url)
+        else:
+            with pytest.raises(OSError):
+                SFrame(url)
+
     def test_s3_sframe_upload_throw(self):
         # s3 only writes when it receives all parts
         # it's sort of atmoic write on file level.