vf_lut: fix simplification / off by 1 error

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_lut.c

@@ -286,9 +286,9 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
             outrow = outrow0;
             for (j = 0; j < w; j++) {
                 switch (lut->step) {
-                case 3:  outrow[3] = tab[3][inrow[3]]; // Fall-through
-                case 2:  outrow[2] = tab[2][inrow[2]]; // Fall-through
-                case 1:  outrow[1] = tab[1][inrow[1]]; // Fall-through
+                case 4:  outrow[3] = tab[3][inrow[3]]; // Fall-through
+                case 3:  outrow[2] = tab[2][inrow[2]]; // Fall-through
+                case 2:  outrow[1] = tab[1][inrow[1]]; // Fall-through
                 default: outrow[0] = tab[0][inrow[0]];
                 }
                 outrow += lut->step;