Avoid memcpy() with same source and destination in pgstat_recv_replslot.

Avoid memcpy() with same source and destination in pgstat_recv_replslot.

Same type of issue as in commit 53d4f5fef and earlier fixes; also
found by apparently-more-picky-than-the-buildfarm valgrind testing.
This one is an oversight in commit 986816750.  Since that's new in
HEAD, no need for a back-patch.