[FFI] Fix global free destruction (#985)

python/tvm/_ffi/_ctypes/function.py

@@ -165,7 +165,7 @@ class FunctionBase(object):
         self.is_global = is_global
 
     def __del__(self):
-        if not self.is_global:
+        if not self.is_global and _LIB is not None:
             check_call(_LIB.TVMFuncFree(self.handle))
 
     def __call__(self, *args):

python/tvm/_ffi/_ctypes/node.py

@@ -44,6 +44,7 @@ class NodeBase(object):
         self.handle = handle
 
     def __del__(self):
+        if _LIB is not None:
             check_call(_LIB.TVMNodeFree(self.handle))
 
     def __getattr__(self, name):