Fix implementation of CustomFieldImpl.compare(Issue, Issue) method

...
SortableCustomField sortable = (SortableCustomField) customFieldType;
FieldConfig c1 = getRelevantConfig(issue1);
FieldConfig c2 = getRelevantConfig(issue2);
if (c1 != null && c1.equals(c2))
{
    return sortable.compare(v1, v2, c1);
}
else
{
    log.info("Sort order for custom field " + this + " for issues " + issue1 + " and " + issue2 + " " +
            "contexts did not match. Sort order may be incorrect");
    return sortable.compare(v1, v2, c1);
}
...

SortableCustomField.compare(Object v1, Object v2, FieldConfig fc)