CLEANUP: hlua: use free_args() to release args arrays

CLEANUP: hlua: use free_args() to release args arrays

Argument arrays used in hlua_lua2arg_check() as well as in the functions
used to call sample fetches and converters were manually released, let's
use the cleaner and more reliable free_args() instead. The prototype of
hlua_lua2arg_check() was amended to mention that the function relies on
the final ARGT_STOP, which is already the case, and the pointless test
for this was removed.