Update ofUtils.cpp

Changed `while (c != EOF);` to `while (!feof(ret));` because in my tests on Raspberry Pi (Raspbian Wheezy) ofSystem() got stuck in the do-while-loop because the last fgetc() didn't return EOF (-1) but 255. The end-of-file-indicator was set though, so feof() works.

I guess feof() would also work on systems where fgetc() correctly returns EOF, so it seems this can be changed generally. If you don't agree, please comment and I will post a commit with the change enclosed in a `#ifdef TARGET_RASPBERRY_PI` condition.