MB-36249: saveDocs: Fix potential divide-by-zero
MB-36249: saveDocs: Fix potential divide-by-zero

Only record flusher write amplification sample if there's a non-zero
number of application bytes written.

Change-Id: I96bc5eb9c7a6cf3f682f4c0bda22f63cf47d9d58
Reviewed-on: http://review.couchbase.org/116726
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: James Harrison <5c25a7b3db09830813688775bfc8e6de736a230c@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine