BUG/MINOR: hlua: segfault when loading the same filter from different contexts

BUG/MINOR: hlua: segfault when loading the same filter from different contexts

Trying to register the same lua filter from global and per-thread context
(using 'lua-load' + 'lua-load-per-thread') causes a segmentation fault in
hlua_post_init().

This is due to a simple copy paste error as we try to print the function
name in the error message (like we do when loading the same lua function
from different contexts) instead of the filter name.

This should be backported up to 2.6.