src: suppress coverity message

Coverity marked a change in 630096bc as a constant expression.
However, on platforms where sizeof(int64_t) > sizeof(size_t),
this should not be the case. This commit flags the comparison
as OK to coverity.

PR-URL: https://github.com/nodejs/node/pull/7587


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>