free accompanying EXIF data when freeing an image

This fixes two memory bugs that only manifest with exif=1 and long-running
slideshows.

* when feh loads an image, it writes exif data to file->ed. Previously, this
  data was never free'd, causing a memory leak on subsequent loads of the same
  file.
* As file->ed is never free'd, the accumulated EXIF data consumes a significant
  amount of memory over time. with slideshow-delay = 10 and two days of
  runtime, feh may exceed 1 GB of memory usage. If the slideshow is so large
  that feh does not encounter the same image twice in this time, this is not
  detected as a memory leak, as each EXIF data chunk is referenced from the
  filelist.

See <https://github.com/derf/feh/issues/553> for details.

Closes #553

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