vinyl: fix potential null-ptr dereference in vy_read_view_merge

The 'copy' ptr is checked for NULL after dereferencing. Found by PVS.

https://jira.vk.team/browse/TNT-698

It can hardly result in a null-ptr dereference, because vy_stmt_dup()
uses malloc(), which doesn't normally fail (instead, the OOM killer just
kills the process if there's a severe shortage of memory, see #3534).
Still, PVS rightfully complains about it so let's fix it.

Fixes commit 902d212b ("vinyl: zap vy_write_iterator->format")

NO_DOC=bug fix
NO_TEST=unreproducible
NO_CHANGELOG=unreproducible