Memory leaks

[jeff-longino]

After some initial testing with 1.9.2 I found memory growth issues.
In Native.cs I changes all calls to the free method
From: leveldb_free(ptr);
To: leveldb_free(ref ptr);

And changed the associated import
[DllImport("libleveldb", CallingConvention = CallingConvention.Cdecl)]
public static extern void leveldb_free(ref IntPtr ptr);

All seems well after these changes. Though it does leave me a little uneasy that I'm not finding anyone else needing to make this change.

I'm using the kkowalczyk-leveldb-6f9f0bca2016 windows port of LevelDB itself.