n-api: fix -Wmaybe-uninitialized compiler warning

Not an actual bug, as far as I can tell, the compiler is simply not
smart enough to figure out that the offending code path isn't reached
with an uninitialized value.

Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/14053


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>