Feature extractor segfault on Linux

[guihao-liang]

Stack trace,

Thread 0x00007fcdc04ec740 (most recent call first):
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/turicreate/extensions.py", line 165 in _run_toolkit_function
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/turicreate/extensions.py", line 181 in <lambda>
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/turicreate/toolkits/_image_feature_extractor.py", line 139 in next_batch
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/turicreate/toolkits/_image_feature_extractor.py", line 179 in extract_features
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/turicreate/toolkits/image_classifier/image_classifier.py", line 839 in _extract_features
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/turicreate/toolkits/image_classifier/image_classifier.py", line 519 in predict
  File "/build/scenario-tests/tests/toolkits/test_image_classifier.py", line 160 in test_sarray
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/python.py", line 184 in pytest_pyfunc_call
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/unittest.py", line 220 in wrapped_testMethod
  File "/usr/lib/python3.6/unittest/case.py", line 605 in run
  File "/usr/lib/python3.6/unittest/case.py", line 653 in __call__
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/unittest.py", line 232 in runtest
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/runner.py", line 135 in pytest_runtest_call
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/runner.py", line 217 in <lambda>
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/runner.py", line 244 in from_call
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/runner.py", line 217 in call_runtest_hook
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/runner.py", line 186 in call_and_report
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/runner.py", line 100 in runtestprotocol
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/runner.py", line 85 in pytest_runtest_protocol
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/main.py", line 272 in pytest_runtestloop
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/main.py", line 247 in _main
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/main.py", line 191 in wrap_session
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/main.py", line 240 in pytest_cmdline_main
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/callers.py", line 187 in _multicall
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 87 in <lambda>
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/manager.py", line 93 in _hookexec
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/pluggy/hooks.py", line 286 in __call__
  File "/build/scenario-tests/venv/lib/python3.6/site-packages/_pytest/config/__init__.py", line 125 in main
  File "/build/scenario-tests/venv/bin/pytest", line 8 in <module>
Segmentation fault (core dumped)
pytest -v -s --junit-xml="toolkits.test_image_similarity.py.xml" "toolkits/test_image_similarity.py"
pytest -v -s --junit-xml="toolkits.test_image_classifier.py.xml" "toolkits/test_image_classifier.py"

might be related to data iterator.