v1.0.0

external_iterator: don't double-close Readers in case of err

Previously, we'd sometimes double-close Readers if
`finishInitializingExternal` returned an error. This is because
we would put all the sstable.Readers in a slice so we could
close them in case of error, but we also called dbi.Close()
which closes them in one return case. Double-closing
would lead to an `inconsistent reference count: -1` panic.

Informs #116566 (will fix when lands in cockroach).