node-api: handle pending exception in cb wrapper

fixes: https://github.com/nodejs/node-addon-api/issues/1025



The functionreference test from the node-api tests
was reporting a failed v8 check when Node.js was compiled
as debug. The failure was because an exception was
pending but the C++ wrappers were returning
a return value that was invalid.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: https://github.com/nodejs/node/pull/39476
Fixes: https://github.com/nodejs/node-addon-api/issues/1025


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>