BUG|MAINT: Fix reference count bugs, init NpyIter strides

Also changes some tests to not use np.empty as to avoid
valgrind spam.

The NpyIter strides are arbitrary, but 0 makes sense, and the
strides are used to decide which transfer function to use, etc.
causing lots of warnings in valgrind.
MAINT: Fix missing decrefs and resolve circular ref in test