MAINT: Initialize strides in NpyIter and silence valgrind

The strides are arbitrary, but 0 makes sense. The empty arrays
cause valgrind to spit many errors, so use filled instead.

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Nothing big, just sets the strides for 0-d iterations (they are never used anyway) and changes a test to not use uninitialized arrays. Makes running the numpy tests in valgrind bearable (with python suppressions, I edited the python supp to match the object malloc stuff very broadly: http://sebastian.sipsolutions.net/python.supp). There is some spam left, but I will wait for memchr fix to see if it stays...