Fix incorrect assignment of InvalidXLogRecPtr to a non-LSN variable.

Fix incorrect assignment of InvalidXLogRecPtr to a non-LSN variable.

pg_logical_slot_get_changes_guts() previously assigned InvalidXLogRecPtr to
the local variable upto_nchanges, which is of type int32, not XLogRecPtr.
While this caused no functional issue since InvalidXLogRecPtr is defined as 0,
it was semantically incorrect.

This commit fixes the issue by updating pg_logical_slot_get_changes_guts()
to set upto_nchanges to 0 instead of InvalidXLogRecPtr.

No backpatch is needed, as the previous behavior was harmless.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Steven Niu <niushiji@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwHKHuR5NGnGxU3+ebz7cbC1ZAR=AgG4Bueq==Lj6iX8Sw@mail.gmail.com