avfilter/vf_blurdetect: fix uninitialized variables and possible div by zero

Found-by: Coverity

libavfilter/vf_blurdetect.c

@@ -138,6 +138,7 @@ static float edge_width(BLRContext *blr, int i, int j, int8_t dir, int w, int h,
     case DIRECTION_VERTICAL:   dX = 0; dY =  1; break;
     case DIRECTION_45UP:       dX = 1; dY = -1; break;
     case DIRECTION_45DOWN:     dX = 1; dY =  1; break;
+    default:                   dX = 1; dY =  1; break;
     }
 
     // determines if search in direction dX/dY is looking for a maximum or minimum
@@ -227,7 +228,7 @@ static float calculate_blur(BLRContext *s, int w, int h, int hsub, int vsub,
                 }
             }
             // if not enough edge pixels in a block, consider it smooth
-            if (block_total_width >= 2) {
+            if (block_total_width >= 2 && block_count) {
                 blks[blkcnt] = block_total_width / block_count;
                 blkcnt++;
             }