Avoid copying byte arrays when just decoding bounds.

Currently, if a caller specifies both "purgeable" and "just decode
bounds" options when passing a byte array to decode, we create an
unnecessary copy of the byte array.  This is probably not common,
but we may as well avoid the copy.

Change-Id: I27e573b0e1fb8f8516729882a84efa02b6da08a5