avfilter/fade: don't allow nb_frames == 0

In filter init, there's a division by nb_frames but value isn't checked.

libavfilter/vf_fade.c

@@ -381,9 +381,9 @@ static const AVOption fade_options[] = {
     { "s",           "Number of the first frame to which to apply the effect.",
                                                     OFFSET(start_frame), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, FLAGS },
     { "nb_frames",   "Number of frames to which the effect should be applied.",
-                                                    OFFSET(nb_frames),   AV_OPT_TYPE_INT, { .i64 = 25 }, 0, INT_MAX, FLAGS },
+                                                    OFFSET(nb_frames),   AV_OPT_TYPE_INT, { .i64 = 25 }, 1, INT_MAX, FLAGS },
     { "n",           "Number of frames to which the effect should be applied.",
-                                                    OFFSET(nb_frames),   AV_OPT_TYPE_INT, { .i64 = 25 }, 0, INT_MAX, FLAGS },
+                                                    OFFSET(nb_frames),   AV_OPT_TYPE_INT, { .i64 = 25 }, 1, INT_MAX, FLAGS },
     { "alpha",       "fade alpha if it is available on the input", OFFSET(alpha),       AV_OPT_TYPE_BOOL, {.i64 = 0    }, 0,       1, FLAGS },
     { "start_time",  "Number of seconds of the beginning of the effect.",
                                                     OFFSET(start_time),  AV_OPT_TYPE_DURATION, {.i64 = 0. }, 0, INT64_MAX, FLAGS },