plug-ins: minor cleaning and fixes.

- Add some parentheses to sizeof (*pointer). Maybe it's just me, but it
  is just a bit clearer to me.
- Free in_raw when out_raw allocation failed.
- Use strrchr() instead of strchr() to get the extension (I had a case
  when testing where the file was inside a folder named similarly with a
  ".hgt" and it would break the comparison test).
- Do a case-insensitive comparison. Even though the original files from
  NASA use lowercase, just to be sure.