fix prefix generation

when performing bitwise operations, java promotes byte's to int's.
this is normally okay except when the value is negative. in that
case, java extends the sign bits. this means that after shifting,
all of the upper bits are most likely going to be all f's. instead,
undo the sign extension by pulling out just the 8 bits we're
interested in.

Change-Id: I89f7244f6b675be2cca4a185a7002b5f8a6eece1