src: guard against overflow in ParseArrayIndex()

ParseArrayIndex() would wrap around large (>=2^32) index values on
platforms where sizeof(int64_t) > sizeof(size_t).  Ensure that the
return value fits in a size_t.

PR-URL: https://github.com/nodejs/node/pull/7497


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>