write_dot_file function doesn't close the file with it opens

[kaustuk-samsung]

https://github.com/FrankBro/be-tree/blob/5c3c3d161e453b725e6ab84f9de351e87c3d4a6d/src/debug.c#L764

Here write_dot_file doesn't close the open file. Due to it the sometimes files contain doesn't get flush out.

Solution: Add the fclose(f); call to close the open file.

PS: I would like to create PR for this.