warning C4716: 'lcurl_easy_cleanup_storage': must return a value

Building with MSVC, I got this warning.

> lceasy.c(203) : warning C4716: 'lcurl_easy_cleanup_storage': must return a value

I fixed it with a `return 0;`, but it can also be fixed my making the function return `void`, so I'm not contributing a pull request at this time; not for such a trivial fix.

There are two call sites, and both ignore the return value.