disambiguate superframe index in vp9_stop_encode()

If the bool-coded partition naturally ends in a byte that matches the
superframe index marker, it could lead to a parse error. This commit
ensures that if such a marker is seen, it is padded out with an
additional zero byte to disambiguate it.

Change-Id: Id977de05745b6fa9ef08afb71e210a2a3ecca02e

vp9/encoder/vp9_boolhuff.c

@@ -53,6 +53,10 @@ void vp9_stop_encode(BOOL_CODER *br) {
 
   for (i = 0; i < 32; i++)
     encode_bool(br, 0, 128);
+
+  // Ensure there's no ambigous collision with any index marker bytes
+  if ((br->buffer[br->pos - 1] & 0xe0) == 0xc0)
+    br->buffer[br->pos++] = 0;
 }