[Build Fix] Implement SoundSourceFFmpeg::parseCoverArt().

Otherwise build fails with:
```
lin64_build/soundsourceffmpeg.o:(.rodata._ZTV17SoundSourceFFmpeg[vtable for SoundSourceFFmpeg]+0x48): undefined reference to `SoundSourceFFmpeg::parseCoverArt()'
collect2: ld returned 1 exit status
scons: *** [lin64_build/mixxx] Error 1
```
A return value of 0 means that the function call succeeded 

https://www.ffmpeg.org/doxygen/2.3/group__lavf__decoding.html#ga10a404346c646e4ab58f4ed798baca32

The standard way to check for this in C is `if (func() != 0)`. It would be nice if you stick with the standard coding style as this makes it easier to read and maintain in the future.