Fix missing numeric casts in cord

Fix missing numeric casts in cord

* cord/cordprnt.c (CORD_vsprintf): Explicitly cast "prec" to unsigned
(to avoid assignment of a signed value to a variable of a bigger
unsigned integer type).
* cord/cordxtra.c (CORD_nul_func, CORD_chars): Cast between pointer
and char via GC_word (instead of long); explicitly cast char to
unsigned char (to avoid a signed value cast to a bigger unsigned one).
* cord/tests/de.c (replace_line): Explicitly cast COLS to unsigned
(when compared to "len" local variable).