Fix path traversal in Album Artist field for CD ripping

For the ALBUMARTIST case in k3bpatternparser, the "/" character
was not being replaced with "_" the way it was for all the other
string fields.  This enables path traversal if the CDDB data has a
"/" in the Album Artist field, creating unwanted extra directories.
This patch fixes it by performing the same replacements on the
album artist as for all the other string fields.