Small amounts of data written from bytes.Buffer can trigger a panic
during cgo argument checking because the pointer points inside the
Buffer struct.

This commit ensures that byte addressing expressions appear in the
arguments to C calls to avoid confusing cgo.  This appears to have
negative effects on benchmark times.  The -test.benchmem flag seems to
indicate that extra allocations are occurring

This saves passing a few unnecessary pointers, passing nil instead.
This should speed up the cgo check by a small amount.

There seems to be a small speed up from this.

…uments