Compactor case clause error

Noticed a shard that could not be compacted. The compaction would start then would crash.
```
{function_clause,[
{couch_bt_engine_compactor,'-copy_docs/4-inlined-2-',[
{3,<<138,72,.....>>},
[],
leaf,
{0,0,[]}
], [{file,"src/couch_bt_engine_compactor.erl"},{line,407}]
},
{couch_key_tree,mapfold_simple,4,[{file,"src/couch_key_tree.erl"},{line,465}]},
{couch_key_tree,mapfold_simple,4,[{file,"src/couch_key_tree.erl"},{line,474}]},
{couch_key_tree,mapfold,3,[{file,"src/couch_key_tree.erl"},{line,458}]},
{couch_bt_engine_compactor,'-copy_docs/4-fun-5-',3,[{file,"src/couch_bt_engine_compactor.erl"},{line,406}]},
{lists,map_1,2,[{file,"lists.erl"},{line,1320}]},
{lists,map_1,2,[{file,"lists.erl"},{line,1320}]},
{lists,map,2,[{file,"lists.erl"},{line,1315}]}
]}
```

That seems to come from:
https://github.com/apache/couchdb/blob/a326cdc53b962789f1d5a069d37177dcffe86cfb/src/couch/src/couch_bt_engine_compactor.erl#L408 where we get `[]` instead of `#leaf{ptr = Sp} = Leaf`.

The only fix so far is to delete the copy and let the other nodes re-replicate the data.

Start with src/couch_bt_engine_compactor.erl around lines 406-408 and trace the copy_docs/4 call shown in the crash. Compare the expected leaf value with the [] value reaching the function_clause, then validate the change by compacting an affected shard and confirming the crash no longer occurs.

Written by the indexing model from the issue text.