message: fix another clippy warning
diff: use std::error::Error;
  use std::fmt;
  use std::io;
  -use std::mem;
  use buffer::{ImageBuffer, Pixel};
  use color;
  @@ -348,7 +347,9 @@ where
  // NOTE: This is potentially dangerous. It would require the signature fn next(&'a mut self) to be safe.
  // error: lifetime of `self` is too short to guarantee its contents can be safely reborrowed...
  - let ptr = unsafe { mem::transmute(tmp) };
  + let ptr = unsafe {
  + &mut *(tmp as *mut <I as GenericImage>::Pixel as *mut <I as GenericImage>::Pixel)
  + };
  let p = (self.x, self.y, ptr);
  