Add cast.

Fixes
root.c:1987:36: error: initialization makes pointer from integer without a cast [-Werror]
with gcc-4.8.3 on NetBSD.
Add casts.

Fixes
error: array subscript has type ‘char’
with gcc-4.8.3 on NetBSD.

See the CAVEATS section in
http://netbsd.gw.com/cgi-bin/man-cgi?ctype+3+NetBSD-current
for an explanation what the problem with "type 'char'" is.