A fix and a test case for Bug#1042738

A fix and a test case for Bug#1042738
"Tree iterator over entire range gives a spurious error"

We didn't look at part_count in num32_key_node_cmp,
and thus could produce wrong comparison result if
part_count was 0.

To be on the safe side, zero key parts even if
part count is 0 (this is what we do elsewhere as well).

Cleanup: make sure you can run box_big/lua.test many times.