data: fix merge operation

lyd_merge_* functions may modify the passed dnode pointer. Make sure to
update the self.cdata pointer after the operation.

Fixes: #49
Signed-off-by: Bill Stephens <william.stephens@megaport.com>

Author: Bill Stephens

libyang/data.py

@@ -552,6 +552,8 @@ def merge(
         if ret != lib.LY_SUCCESS:
             raise self.context.error("merge failed")
 
+        self.cdata = node_p[0]
+
     def iter_tree(self) -> Iterator["DNode"]:
         n = next_n = self.cdata
         while n != ffi.NULL: