dgram: fix possibly deoptimizing use of arguments

This commit adds a guard against an out of bounds access of
arguments, and replaces another use of arguments with a named
function parameter.

Refs: https://github.com/nodejs/node/issues/10323
PR-URL: https://github.com/nodejs/node/pull/11242


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>