fix HFE disk write

[kkaempf]

this fixes https://mametesters.org/view.php?id=7526

src/lib/formats/hxchfe_dsk.cpp reproducibly creates a SEGV when writing HFE files. This is caused by an endless loop in hfe_format::generate_hfe_bitstream_from_track, writing beyond end-of-array at

mame/src/lib/formats/hxchfe_dsk.cpp

Line 683 in 140ba51

cylinder_buffer[offset++] = current;

The endless loop is caused by re-setting cur_pos at two occasions and re-setting the loop variable.

This fix drops the reset of cur_pos resulting in a successful write of a mixed-density (track0 sd, others dd) HFE image.