Structure blocks don't overwrite existing files when saved

If an existing structure file is overwritten by a new structure (i.e., by saving a new structure with the same name), but the new structure is not loaded before the player logs off, the saved structure will revert to the original version.

Steps to reproduce:
1. Save any structure as "test1"
2. Load "test1"
3. Save a new, different structure as "test1"
4. Relog
5. Load "test1" - the original structure will be loaded instead of the modified structure, even though the modified structure was saved before logging off .