Fix a bug in generate_evasions()

Fix a bug in generate_evasions()

Introduced in the patch "movegen: prefer (*mlist++) to mlist[n++]"

This was nasty because due to a mismerge the repo in one PC had the bug,
but the testing one did not, so I had non reproducible results according
to which machine I used for testing.

This hopefully closes a more then one week regression that made me go crazy!

It was found by accident comparing, for other reasons, the sources of the
two PC's.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>