Fix incorrect assertion on number of array dimensions.
This has been wrong ever since the support for multi-dimensional
arrays as PL/python function arguments and return values was
introduced in commit
94aceed317.
Backpatch-through: 10
Discussion: https://www.postgresql.org/message-id/
61647b8e-961c-0362-d5d3-
c8a18f4a7ec6%40iki.fi