Update to 2.18.3

http://git.finalrewind.org/feh/plain/ChangeLog:
Tue, 04 Apr 2017 21:22:16 +0200  Daniel Friesel <derf+feh@finalrewind.org>

* Release v2.18.3
    * Fix double-free/OOB-write in E17 IPC. This only affects the
      background setting options and requires a malicious X11 app to run
      alongside feh and pretend to be an E17 window manager.
      (patch by Tobias Stoeckmann)
    * Fix image-specific format specifiers not being updated correctly in
      thumbnail mode window titles
    * Fix memory leak when closing images opened from thumbnail mode
    * Fix a possible out of bounds read caused by an unterminated string when
      using --output to save images in long paths. (patch by Tobias Stoeckmann)
    * Fix out of bounds read/write when handling empty or broken caption files.
      (patch by Tobias Stoeckmann)
    * Fix memory leak when saving a filelist or image whose target filename
      already exists. (patch by Tobias Stoeckmann)

(szptvlfn)