can trigger an abort when compiling with debug=yes, and using 7men TB.
The assert should check that less than 8 pieces are in the key for each
side, matching the assumption that underlies the FEN string construction.
Also take explicitly care of a 'v' character in material strings.
Fixes an issue reported in the forum:
https://groups.google.com/d/msg/fishcooking/yoVC7etIpz0/7mS7ntZMBAAJ