avfilter/avf_avectorscope: Assert that format is valid

This should help coverity realize that src[] is inited
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/avf_avectorscope.c

@@ -267,6 +267,8 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *insamples)
             src[0] = samplesf[0];
             src[1] = samplesf[1];
             break;
+        default:
+            av_assert2(0);
         }
 
         switch (s->scale) {