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.