ubus: Fix imbalance in lua stack push/pop of values.

ubus: Fix imbalance in lua stack push/pop of values.

The lua getglobal and rawgeti both push a value onto the lua stack,
but they weren't being removed by the ubus_method_handler function,
thus corrupting the lua stack.

In case the specified method wasn't a function, the stack was also
corrupted as the method name remained on the stack.

Signed-off-by: Karl Vogel <karl.vogel@gmail.com>