Fix for failing to zero-initialize Win32API structure.

If api_init raises before assigning to the library member, api_free will attempt to FreeLibrary an uninitialized value.
This will result in undefined behavior (an access violation, more often than not).
With a debugger attached to ruby.exe, it's easy to reproduce with: `Win32::API.new('foo', 'V', 'V', 'foo')`, without a foo.dll on the load path, and waiting for the API object to be collected.