Make BufferedImage transient in ImageIOReader (Fix #11334)

To prevent unnecessarily large Memoizer files in ImageIOReader
subclasses, the `img` field is marked transient. Access must
now be protected by a null check with a call to `initImage()`
if the field is null.
Merge pull request #626 from joshmoore/11334-jpg-memo

Make BufferedImage transient in ImageIOReader (Fix #11334)