expr: fix missing FMA case label
expr: fix missing FMA case label

Fortunately, this accidental missed case label shouldn't affect
much as the optimizer shouldn't generate FMA on constants.

Signed-off-by: akarin <042dc4512fa3d391c5170cf3aa61e6a638f84342@akarin.info>

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