upvalue paranoia

[Rebel Neurofog <rebelneurofog@...>]

Hi!

I use lua_pushcclosure () to push C function with user data attached to it.
Currently I check it as an ordinary C function argument.
My rational mind told me that I don't have to, but my paranoia have a
different point of view.

So the question is (I feel pretty stupid asking it):
may I always trust in upvalues or there are some states (upvalues
aren't collected, right?)
when I may get NULL from lua_touserdata () instead?