Possible memory leak when creating a region

I just noticed a possible memory leak:
https://github.com/awesomeWM/oocairo/blob/2aba6f923a04376397f85eda0e76e612c236860d/obj_region.c#L40-L50
The `calloc` result is leaked if an error happens (either via `luaL_error` or via `lua_next` doing a longjmp).

Start in obj_region.c at lines 40-50, where the region is created and the calloc result is handled. Trace the luaL_error and lua_next failure paths; done means those paths no longer leave the allocated region leaked, with the normal creation path still working.

Written by the indexing model from the issue text.