fix side-effect in get_qual_for_list()

Hi,

While working on one of the crash reported on default partition for list
partitioning table[1] I found some strange behavior in get_qual_for_list()
while
I tried to call it from the new code I wrote for default partition.

After debugging, I noticed that the function get_qual_for_list() is
implicitly
manipulating the (PartitionBoundSpec) spec->listdatums list. AFAICU, this
manipulation is needed just to construct a list of datums to be passed to
ArrayExpr, and this should be done without manipulating the original list.
The function name is get_qual_for_list(), which implies that this function
returns something and does not modify anything.

I have made this change in attached patch, as I think this is useful for
future
developments, as there may be a need in future to call get_qual_for_list()
from
other places, and the caller might not expect that PartitionBoundSpec gets
modified.

PFA.

[1] *https://www.postgresql.org/message-id/flat/CAOgcT0PLPge%3D5U6%3DGU5SnC3_8yutCbWWOiUva3Cw94M9zpbvgQ%40mail.gmail.com
<https://www.postgresql.org/message-id/flat/CAOgcT0PLPge%3D5U6%3DGU5SnC3_8yutCbWWOiUva3Cw94M9zpbvgQ%40mail.gmail.com>*

Regards,
Jeevan Ladhe