Query time reduce failed

{markdown}
We have logs of a case where the reduce is failing on query time:

```
[couchdb:error,2016-04-14T12:19:24.323,ns_1@172.23.105.60:<0.10936.1444>:couch_log:error:44]Uncaught error in HTTP request: {error,{badmatch,{[]}}}

Stacktrace: [{couch_set_view_mapreduce,'builtin_stats/2-fun-1',2,
[{file, "/home/buildbot/buildbot_slave/ubuntu-1004-x64-30x-builder/build/build/couchdb/src/couch_set_view/src/couch_set_view_mapreduce.erl"},
{line,319}]},
{lists,foldl,3,[{file,"lists.erl"},{line,1248}]},
{couch_set_view_mapreduce,builtin_stats,2,
[{file, "/home/buildbot/buildbot_slave/ubuntu-1004-x64-30x-builder/build/build/couchdb/src/couch_set_view/src/couch_set_view_mapreduce.erl"},
{line,314}]},
{couch_set_view_mapreduce,builtin_reduce,4,
[{file, "/home/buildbot/buildbot_slave/ubuntu-1004-x64-30x-builder/build/build/couchdb/src/couch_set_view/src/couch_set_view_mapreduce.erl"},
{line,261}]},
{couch_set_view_mapreduce,'rereduce/2-fun-0',3,
[{file, "/home/buildbot/buildbot_slave/ubuntu-1004-x64-30x-builder/build/build/couchdb/src/couch_set_view/src/couch_set_view_mapreduce.erl"},
{line,164}]},
{lists,zipwith,3,[{file,"lists.erl"},{line,436}]},
{couch_set_view_mapreduce,rereduce,2,
[{file, "/home/buildbot/buildbot_slave/ubuntu-1004-x64-30x-builder/build/build/couchdb/src/couch_set_view/src/couch_set_view_mapreduce.erl"},
{line,162}]},
{mapreduce_view,'setup_views/5-fun-1',7,
[{file, "/home/buildbot/buildbot_slave/ubuntu-1004-x64-30x-builder/build/build/couchdb/src/couch_set_view/src/mapreduce_view.erl"},
{line,489}]}]
```

The logs are from MB-19187:
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.105.60.zip

Logs from the rest of the cluster:
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.105.61.zip
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.105.62.zip
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.105.63.zip
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.105.83.zip
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.106.14.zip
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.106.213.zip
https://s3.amazonaws.com/scalability-mcafee/MB-19187/during_initiate_indexing/collectinfo-2016-04-14T194122-ns_1%40172.23.106.96.zip
{markdown}