dfhack printall_recurse unsafely recurses through untagged unions #5986

printall_recurse will, while recursing from its starting origin, traverse through all of the fields of an untagged union, whether or not those fields contain valid data. for a tagged union, it's safe to traverse the field corresponding to the tag, but for an untagged union the recursive print can't tell which field (if any) contains valid data, and thus recursion must not proceed into the union fields