Eliminate warnings detected by Cppcheck in cord de[_win]

Eliminate warnings detected by Cppcheck in cord de[_win]

* cord/tests/de.c (prune_map): Remove redundant first check of map for
NULL (as "map" variable has already been dereferenced at the point of
the check).
* cord/tests/de.c (replace_line): Do not define "len" local variable
if not used; eliminate duplicate CORD_len(s) call.
* cord/tests/de_win.c (WndProc): Cast char_width and char_height
global variables to unsigned int in division of unsigned xpos, ypos
local variables.