Offset in first extended partition not handled correctly

[rubenbe]

I was writing this unusual config, and it brought up an issue in genimage. Part4 is at the correct location, but the ERB of part3 does not point correctly to part4. Tested on 207ce8df4011ef084d5c178d5be30c9c495aa94d

image test.hdimage {
    hdimage {
        extended-partition = 3
    }
    partition part1 {
        offset = 128K
        size =  256K
        partition-type = 0x83
    }
    partition part2 {
        size = 3072K
        partition-type = 0x83
    }
    partition part3 {
        offset = 3686400
        size = 8126464
        partition-type = 0x83
    }
    partition part4 {
        offset = 11821056
        size = 16252928
        partition-type = 0x83
    }
}

[michaelolbrich]

Fixed in #149.