message: fix 0-sized getrandom
diff: @@ -241,9 +241,11 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a + 'mir>: crate::MiriEvalContextExt<'
  // neither of which have any effect on our current PRNG
  let _flags = this.read_scalar(args[3])?.to_i32()?;
  + if len > 0 {
  let data = gen_random(this, len as usize)?;
  this.memory_mut().get_mut(ptr.alloc_id)?
  .write_bytes(tcx, ptr, &data)?;
  + }
  this.write_scalar(Scalar::from_uint(len, dest.layout.size), dest)?;
  }
  @@ -769,9 +771,11 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a + 'mir>: crate::MiriEvalContextExt<'
  let ptr = this.read_scalar(args[0])?.to_ptr()?;
  let len = this.read_scalar(args[1])?.to_usize(this)?;
  + if len > 0 {
  let data = gen_random(this, len as usize)?;
  this.memory_mut().get_mut(ptr.alloc_id)?
  .write_bytes(tcx, ptr, &data)?;
  + }
  this.write_scalar(Scalar::from_bool(true), dest)?;
  }
  