log: Don't overwrite valid tags
log: Don't overwrite valid tags

If a tag of 0 is passed into the logger and an existing callsite
is found with a non-zero tag, the don't overwrite the existing tag.

Signed-off-by: Christine Caulfield <992cedbe1eac7dbaff482fcb880a50b57e6b8062@redhat.com>

ClusterLabs/libqb,ClusterLabs/libqb,ClusterLabs/libqb,kgaillot/libqb,kgaillot/libqb,kgaillot/libqb