Fix recently-introduced crash in array_contain_compare().
Silly oversight in commit
1dc5ebc9077ab742079ce5dac9a6664248d42916:
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.