test: fix build warning in addons-napi\test_object
Fix a build warning in the test, reported at https://github.com/nodejs/node/pull/13250#issuecomment-305670009
Note this is plain C code, so C++ style casts cannot be used.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
test