libtracker-common: Fix media art paths when artist is not known
libtracker-common: Fix media art paths when artist is not known

Correct behaviour is prefix-md5sum(title)-md5sum( ) but previously
we were generating prefix-md5sum( )-md5sum(title)

hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker