graphics/rawtherapee: fix SIGSEGV in RGB_denoise
- Bug report https://github.com/Beep6581/RawTherapee/issues/5796
- Cherry-pick https://github.com/Beep6581/RawTherapee/commit/5239e61946870a5405e94b138b4196fc35159ab1
- Bump PORTREVISION
Obtained from: Ingo Weyrich/upstream Github repo
MFH: 2020Q2 (blanket approval for stability fixes)