Fix for -d option

I ran into the problem, that the -d option (e.g. `recoverjpeg -d "%05d" test.img`) was not working on my system (Debian & cygwin). I always got this error message: `recoverjpeg: unable to create directory 00000 (17 (File exists))`. That's because recoverjpeg wants to create the directory regardless whether it was already created before or not.

I've added a check in line 185; the directory is now created only when the directory does not exist.

This fixed the issue and recoverjpeg was able to recover over 195.000 files from an image of a broken hard disk :-)

I've tested the code on Debian 6.0.6 and cygwin 1.7.17 and it worked on both systems.