get_label_range_var(args) returns new RangeVar pointer

The name of the function:
```
RangeVar *get_label_range_var(char *graph_name, Oid graph_oid, char *label_name)
```
in `ag_label.c` is misleading.

This function generates a new RangeVar pointer rather than returning the one that was first generated when a label was created, I believe its name should be changed to something like "`create_label_range_var`" or "`generate_label_range_var`".

Also, the function `get_label_range_var` should be fixed to return the correct pointer.