Fix recently-introduced crash in array_contain_compare().

Silly oversight in commit 1dc5ebc9:
when array2 is an expanded array, it might have array2->xpn.dnulls equal
to NULL, indicating the array is known null-free.  The code wasn't
expecting that, because it formerly always used deconstruct_array() which
always delivers a nulls array.

Per bug #13334 from Regina Obe.