libtracker-extract: Don't assume ':' exists in every path we iterate
libtracker-extract: Don't assume ':' exists in every path we iterate

This makes sure we don't accidentally return g_strdup(NULL + 1) when iterating
XMP data

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