Fix macOS compiler warning in HdrHistogram::makeIterator
Fix macOS compiler warning in HdrHistogram::makeIterator

Fix compiler warning:
"comparison of unsigned expression < 0 is always false"
Which was ocouring to to an incorrect cast from int64_t to uint64_t.

Change-Id: I6aedab1500ac1f886e8dc73ffc933ab44730c5d2
Reviewed-on: http://review.couchbase.org/119944
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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