avfilter/vf_v360: Assert that vf was initialized

Maybe helps: CID1504571 Uninitialized scalar variable

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f802d65de0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_v360.c

@@ -3787,6 +3787,8 @@ static int barrelsplit_to_xyz(const V360Context *s,
         case 3: // back bottom
             vf = (y * 2.f - 1.5f) / scaleh + 3.f - facef;
             break;
+        default:
+            av_assert0(0);
         }
         l_x = (0.5f - uf) / scalew;
         l_y =  0.5f * dir_vert;