make gen-character more robust
make gen-character more robust

code-char is allowed to return NIL for a given code.
gen-character now checks for this, so that it will always
return a character.

darcs-hash:20080320094911-7ef6b-2d018cccef118c4f714ba775a1a1f8189a47d3d0.gz

kmizumar/fiveam