Fix dialyzer warnings

Erlang 17.0 emits two dialyzer warnings:

    couch_view_merger.erl:776: The created fun has no local return
    couch_view_merger.erl:1097: The created fun has no local return

This commit fixes those.

Change-Id: I7e90998818f3ce8fa503a09b09f7aada5b0d7ccf
Reviewed-on: http://review.couchbase.org/37122
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>