src: fix bad sizeof expression
It was computing the size of the pointer, not the size of the pointed-to object.
Introduced in commit 727b2911 ("src,dns: refactor cares_wrap to avoid global state".)
CI: https://ci.nodejs.org/job/node-test-pull-request/11416/ (jenkins issue)
CI: https://ci.nodejs.org/job/node-test-pull-request/11418/