`array_position...()` causes SIGSEGV

Hello hackers,

I met SIGSEGV when using `array_position()` with record type
arguments, so I've written a patch which corrects this problem. It
seems that `array_position...()` sets wrong memory context for the
cached function (in this case `record_eq()`) which is used to find a
matching element.

The problem is reproducable with the following query.

SELECT array_position(ids, (1, 1))
FROM (VALUES (ARRAY[(0, 0)]), (ARRAY[(1, 1)])) AS _(ids);