lavc/vp9: clarify inv_recenter_nonneg

Ends up identical to Libav.

libavcodec/vp9.c

@@ -261,7 +261,11 @@ static av_always_inline int get_sbits_inv(GetBitContext *gb, int n)
 
 static av_always_inline int inv_recenter_nonneg(int v, int m)
 {
-    return v > 2 * m ? v : v & 1 ? m - ((v + 1) >> 1) : m + (v >> 1);
+    if (v > 2 * m)
+        return v;
+    if (v & 1)
+        return m - ((v + 1) >> 1);
+    return m + (v >> 1);
 }
 
 // differential forward probability updates