rpcrt4: Mark the buffer as invalid in NdrProxyFreeBuffer().

dlls/rpcrt4/cproxy.c

@@ -586,7 +586,7 @@ void WINAPI NdrProxyFreeBuffer(void *This,
   {
     IRpcChannelBuffer_FreeBuffer(pStubMsg->pRpcChannelBuffer,
                                  (RPCOLEMESSAGE*)pStubMsg->RpcMsg);
-    pStubMsg->fBufferValid = TRUE;
+    pStubMsg->fBufferValid = FALSE;
   }
   IRpcChannelBuffer_Release(pStubMsg->pRpcChannelBuffer);
   pStubMsg->pRpcChannelBuffer = NULL;