message: remove no-longer-needed zero checks
diff: @@ -175,10 +175,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  .to_machine_usize(&*this.tcx)?
  .min((1 << (ptr_size - 1)) - 1) // max value of target `isize`
  .min(isize::max_value() as u64);
  - // Reading zero bytes should not change `buf`.
  - if count == 0 {
  - return Ok(0);
  - }
  +
  let fd = this.read_scalar(fd_op)?.to_i32()?;
  let buf = this.read_scalar(buf_op)?.not_undef()?;
  @@ -231,10 +228,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  .to_machine_usize(&*this.tcx)?
  .min((1 << (ptr_size - 1)) - 1) // max value of target `isize`
  .min(isize::max_value() as u64);
  - // Writing zero bytes should not change `buf`.
  - if count == 0 {
  - return Ok(0);
  - }
  +
  let fd = this.read_scalar(fd_op)?.to_i32()?;
  let buf = this.read_scalar(buf_op)?.not_undef()?;
  