vinyl: do not log dump if index was dropped

An index can be dropped while a memory dump is in progress. If the vinyl
garbage collector happens to delete the index from the vylog by the time
the memory dump completes, the dump will log an entry for a deleted
index, resulting in an error next time we try to recover the vylog,
like:

```
ER_INVALID_VYLOG_FILE: Invalid VYLOG file: Run 2 committed after deletion
```

or

```
ER_INVALID_VYLOG_FILE: Invalid VYLOG file: Deleted range 9 has run slices
```

We already fixed a similar issue with compaction in commit 29e2931c
("vinyl: fix race between compaction and gc of dropped LSM"). Let's fix
this one in exactly the same way: discard the new run without logging it
to the vylog on a memory dump completion if the index was dropped while
the dump was in progress.

Closes #10277

NO_DOC=bug fix