sys/vfs/hammer2: Don't use 1 byte size for 0 radix bits

sys/vfs/hammer2: Don't use 1 byte size for 0 radix bits

Not using 1 byte size for 0 radix bits is part of ondisk spec.
This may not be expected to happen in these cases, but other ones
always consider 0 radix or assert radix != 0.

--
 * WARNING! A radix of 0 (such as when data_off is all 0's) is a special
 *	    case which means no data associated with the blockref, and
 *	    not the '1 byte' it would otherwise calculate to.