lua plugin: cleanup properly when shutting down

lua plugin: cleanup properly when shutting down

Found while running collectd -T under valgrind:

==25971== 75 (64 direct, 11 indirect) bytes in 1 blocks are definitely lost in loss record 154 of 274
==25971==    at 0x483AB1A: calloc (vg_replace_malloc.c:762)
==25971==    by 0x4854009: lua_cb_register_read (lua.c:288)
==25971==    by 0x4FBE886: ???
==25971==    by 0x4FD4900: ???
==25971==    by 0x4FBEAD7: ???
==25971==    by 0x4FBEB04: ???
==25971==    by 0x4FBDF16: ???
==25971==    by 0x4FBEE5E: ???
==25971==    by 0x4FB53ED: ???
==25971==    by 0x48536E7: lua_script_load (lua.c:465)
==25971==    by 0x485387E: lua_config_script (lua.c:532)
==25971==    by 0x485395E: lua_config (lua.c:557)