n-api: napi_get_cb_info should fill array

When the number of args requested is greater than the actual number of
args supplied to the function call, the remainder of the args array
should be filled in with `undefined` values. Because of this bug, the
remainder of the array was left uninitialized, which could cause a
crash.

Refer to the documentation for the `argv` parameter at
https://github.com/nodejs/node/blob/master/doc/api/n-api.md#napi_get_cb_info

PR-URL: https://github.com/nodejs/node/pull/12863


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