Fix double table_close of sequence_rel in copy_sequences().

Fix double table_close of sequence_rel in copy_sequences().

sequence_rel was declared at batch scope, so when a row is skipped due to
concurrent drop or insufficient privileges, the end-of-row cleanup closes
the stale pointer from the previous row, tripping the relcache refcount
assertion.

Move sequence_rel inside the per-row loop.

Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Discussion: https://postgr.es/m/CAJTYsWWOuw-yfmzotV4jCJ6LLxEsb=STLcGtDYXOxRcU9Te3Pw@mail.gmail.com