BUG/MINOR: hlua: improper lock usage in hlua_filter_callback()

BUG/MINOR: hlua: improper lock usage in hlua_filter_callback()

In hlua_filter_callback(), some lua stack work is performed under
SET_SAFE_LJMP() guard which also takes care of locking the hlua context
when needed. However, a lua_gettop() call is performed out of the guard,
thus it is unsafe in multithreading context if the script is loaded using
'lua-load' because in this case the main lua stack is shared between
threads and each access to a lua stack must be performed under the lock,
thus we move lua_gettop() call under the lock.

It should be backported up to 2.6.