BUG/MINOR: hlua: prevent function and table reference leaks on errors

BUG/MINOR: hlua: prevent function and table reference leaks on errors

Several error paths were leaking function or table references.
(Obtained through hlua_checkfunction() and hlua_checktable() functions)

Now we properly release the references thanks to hlua_unref() in
such cases.

This commit depends on "MINOR: hlua: add simple hlua reference handling API"

This could be backported in every stable versions although it is not
mandatory as such leaks only occur on rare error/warn paths.
[prior to 2.5 lua filter API did not exist, the hlua_register_filter()
part should be skipped]