MB-6217: avoid division by zero while finalizing indexing task
MB-6217: avoid division by zero while finalizing indexing task

I don't know how, but it appears that sometimes total_changes can
indeed be 0.

I've hit this badarith a bit too often on my rebalance benchmark runs
before patch, and seen no problems after.

Change-Id: I181fafef5cc32d7b20ab48ef7fb08ceeefc9d5c6
Reviewed-on: http://review.couchbase.org/22120
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

ceejatec/ns_server,daverigby/ns_server,nimishzynga/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,membase/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,membase/ns_server,ceejatec/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,mschoch/ns_server,daverigby/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,membase/ns_server,vzasade/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,ceejatec/ns_server,daverigby/ns_server,vzasade/ns_server,membase/ns_server