[kpartx] bug fixes for dos extended partition [3/4]

[kpartx] bug fixes for dos extended partition [3/4]

in the for loop in read_extended_partition,
'here' is updated if the next extended partition is found.

However, the 'here' value should be updated in the next while loop.
Otherwise it's possible that following partition table
entry refers updated value and generate wrong mapping.

I'm not sure if there is any partitioning tool which can generate
such table. But it's safer to fix this.

Jun'ichi Nomura, NEC