Fix async view fetch

Fix case in async fetch with include_docs, when last chunk arrives
after last doc fetched.

Change-Id: Ia022d69c9a82ab80be08ec022d623ba510f5d485
Reviewed-on: http://review.couchbase.org/23991
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>