Expr: Fix another bug in the constant folding
Expr: Fix another bug in the constant folding

Dup was mistakenly classified as an operator with one operand
instead of zero.

Kamekameha/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth,vapoursynth/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth