fix callback

[vilenkamalov]

make callback working as intended

[robertlzj]

Hi, test with test_alien.lua will fail on each callback

H:\TDDOWNLOAD\alien\alien-0.7.1\tests\test_alien.lua:48: bad argument #4 to 'callback' (string expected, got userdata)
stack traceback:
	[C]: in function 'alien_c.callback'
	H:\TDDOWNLOAD\alien\alien-0.7.1\tests\test_alien.lua:48: in main chunk
	[C]: in ?

where the code is local cb = alien.callback(callback, "int", "int"). Lua 5.3 x32, Win10