Fix typo in tts_virtual_copyslot.

The code used the destination slot's natts where it intended to
use the source slot's natts.  Adding an Assert shows that there
is no case in "make check-world" where these counts are different,
so maybe this is a harmless bug, but it's still a bug.

Takayuki Tsunakawa

Discussion: https://postgr.es/m/0A3221C70F24FB45833433255569204D1FD34C0E@G01JPEXMBYT05