Change timestamp ineffective

[anmin]

Changing timestamp involving EXIF is ineffective when file with no EXIF is also selected.

- Selecting a group of image files, where there is a (or more) file "F" with no EXIF (for example, an AVI file, not even a JPG/TIF file).
- Using tool->Change timestamp, and configuring using EXIF time and date (in "Use" block).
- The timestamps of selected images before file "F" are updated, but those after file "F" are not.