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.

PR-URL: nodejs#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>

src/node_api.cc

@@ -2326,7 +2326,7 @@ napi_status napi_instanceof(napi_env env,
   }
 
   if (env->has_instance_available) {
-    napi_value value, js_result, has_instance = nullptr;
+    napi_value value, js_result = nullptr, has_instance = nullptr;
     napi_status status = napi_generic_failure;
     napi_valuetype value_type;