Fix possible crash if snapshot and DB take part in cyclic gargabe collection

The cyclic garbage collector may choose this snappshot object to break the
cycle. In that case tp_clear will remove the reference to self.db.
So if __dealloc__ of the snapshot is called, self.db is not valid anymore

rocksdb/_rocksdb.pyx

@@ -1309,6 +1309,7 @@ cdef class DB(object):
         def __get__(self):
             return self.opts
 
+@cython.no_gc_clear
 @cython.internal
 cdef class Snapshot(object):
     cdef const snapshot.Snapshot* ptr