String index out of range when doing a complex inverse FFT

When doing an inverse **complex** FFT on a stack with a short name, the following error appears: _java.lang.StringIndexOutOfBoundsException: String index out of range: -9_. This has to do with line 553 in FFT.java: _String name = WindowManager.getUniqueName(imp.getTitle().substring(10))_.

Steps to reproduce:
- Open an image
- Do a **complex** FFT
- Rename the stack to something with <10 characters
- Do an inverse FFT

Workaround: rename the stack to something with >10 characters

Start at line 553 of FFT.java and reproduce the inverse complex FFT after renaming a stack to fewer than 10 characters. The fix is done when this workflow no longer raises StringIndexOutOfBoundsException for a short stack name.

Written by the indexing model from the issue text.