avfilter/avf_avectorscope: assert that variables are initialized by switch()

Silences: CID1351390
Silences: CID1351391
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/avf_avectorscope.c

@@ -300,6 +300,8 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *insamples)
             prev_y = y;
         }
         break;
+    default:
+        av_assert0(0);
     }
 
     s->prev_x = x, s->prev_y = y;