MB-100: fix a couple of compilation warnings

The warnings of comparison between signed and unsigned integers
come out everytime I recompile ep_engine code, not critical but
a bit annoying,  thus probably it is better to fix it.

Change-Id: I82525f68edabbeec7060bbe986a40a1d3a6c9ce4
Reviewed-on: http://review.couchbase.org/34568
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>