BUG/MINOR: hlua: fixing ambiguous sizeof in hlua_load_per_thread

BUG/MINOR: hlua: fixing ambiguous sizeof in hlua_load_per_thread

As pointed out by chipitsine in GH #1879, coverity complains
about a sizeof with char ** type where it should be char *.

This was introduced in 'MINOR: hlua: Allow argument on
lua-lod(-per-thread) directives' (ae6b568)

Luckily this had no effect so far because on most platforms
sizeof(char **) == sizeof(char *), but this can not be safely
assumed for portability reasons.

The fix simply changes the argument to sizeof so that it refers to
'*per_thread_load[len]' instead of 'per_thread_load[len]'.

No backport needed.