fix tautological compare error

In case of userdata I=0, size_t i(0) is never less than 0, causing a tautological-compare warning.

sol/function_types.hpp:262:38: error: comparison of unsigned expression < 0 is always
      false [-Werror,-Wtautological-compare]

FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610
Target: x86_64-unknown-freebsd10.0
Thread model: posix