avfilter/vf_despill: fix assigment

Reported-by: Moritz Barsnick
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/vf_despill.c

@@ -161,7 +161,7 @@ static const AVOption despill_options[] = {
     {   "blue",     "bluescreen",              0,                   AV_OPT_TYPE_CONST,   {.i64=1},     0,   0, FLAGS, "type" },
     { "mix",        "set the spillmap mix",    OFFSET(spillmix),    AV_OPT_TYPE_FLOAT,   {.dbl=0.5},   0,   1, FLAGS },
     { "expand",     "set the spillmap expand", OFFSET(spillexpand), AV_OPT_TYPE_FLOAT,   {.dbl=0},     0,   1, FLAGS },
-    { "red",        "set red scale",           OFFSET(bluescale),   AV_OPT_TYPE_FLOAT,   {.dbl=0},  -100, 100, FLAGS },
+    { "red",        "set red scale",           OFFSET(redscale),    AV_OPT_TYPE_FLOAT,   {.dbl=0},  -100, 100, FLAGS },
     { "green",      "set green scale",         OFFSET(greenscale),  AV_OPT_TYPE_FLOAT,   {.dbl=-1}, -100, 100, FLAGS },
     { "blue",       "set blue scale",          OFFSET(bluescale),   AV_OPT_TYPE_FLOAT,   {.dbl=0},  -100, 100, FLAGS },
     { "brightness", "set brightness",          OFFSET(brightness),  AV_OPT_TYPE_FLOAT,   {.dbl=0},   -10,  10, FLAGS },