jvshost: Fix size of encoded packet with escaped bytes

[987123879113]

The size of the encoded packet returned by the encode function was off by the number of bytes that were escaped. This issue is only seen when a raw packet includes bytes that need to be escaped (0xd0 or 0xe0).

The add variable is added to the output size variable after the loop but since the add variable is directly decremented every time a byte is escaped in the loop, the final size += add turns into size += 0.