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|BUG: Some small refcounting fixes
BUG: Fix clongdouble type in scalar type selection
BUG: Further reference count issues in __int__ and friends
TST: Initialize some arrays to to avoid valgrind warnings
Merge pull request #3965 from seberg/valgrind

MAINT: Initialize strides in NpyIter and silence valgrind