graphics/rawtherapee: fix another potential SIGSEGV in RGB_denoise

Manually cherry-pick and merge 6c1055f from upstream Git into existing
files/patch-rtengine_FTblockDN.cc

Bump PORTREVISION again.

https://github.com/Beep6581/RawTherapee/issues/5796#issuecomment-643774987

Reported by: Ingo Weyrich
Obtained from: Ingo Weyrich
MFH: 2020Q2 (blanket approval for small stability fixes)