avfilter/vf_v360: Add assert to suppress Coverity false positives

Should fix many Coverity false positives, namely #1457947-#1457994
as well as #1461195-#146210.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/vf_v360.c

@@ -286,6 +286,8 @@ static int remap##ws##_##bits##bit_slice(AVFilterContext *ctx, void *arg, int jo
     const AVFrame *in = td->in;                                                                            \
     AVFrame *out = td->out;                                                                                \
                                                                                                            \
+    av_assert1(s->nb_planes <= AV_VIDEO_MAX_PLANES);                                                       \
+                                                                                                           \
     for (int stereo = 0; stereo < 1 + s->out_stereo > STEREO_2D; stereo++) {                               \
         for (int plane = 0; plane < s->nb_planes; plane++) {                                               \
             const unsigned map = s->map[plane];                                                            \