arrow

ARROW-726: [C++] Fix segfault caused when passing non-buffer object to arrow::py::PyBuffer

This leads to calling `Py_DECREF` on a null pointer

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #459 from wesm/ARROW-726 and squashes the following commits:

a764134 [Wes McKinney] Fix segfault caused when passing non-buffer object to arrow::py::PyBuffer. Fix some compiler warnings