Seg Fault in Fast Julia Mode when user formula divides by zero

[JohnDorsey]

Division by zero in the user formula causes a segmentation fault if Fast Julia Mode is enabled.

Steps to reproduce the behavior:

1. either set user formula to "z^2+c+c/z" and set user initialization to "0", or simply set user formula to "z^2+c+c/0".
2. click Fractal > Fast Julia Mode.
3. "Segmentation Fault (core dumped)"
   The order of steps 1 and 2 does not matter.

Desktop:

• OS: Xubuntu 20.04.3 LTS x86_64, Linux kernel 5.13.0-35-generic.
• Browser: Firefox 98.0.2 is default.
• Version: XaoS 4.2.1 (linux x86_64).

Additional context:
The crash seems reliable, and is not avoided by disabling extra features like periodicity checking, dynamic guessing, or dynamic resolution. This problem was first discovered in xaos version (3.5+ds1-3.1build2). I updated, and tested it in the latest version before making this report.