comms/xmorse: Fix build with Clang 6

Codebox.cxx:122:16: error: non-constant-expression cannot be narrowed from type 'int' to 'char' in initializer list [-Wc++11-narrowing]

char s[] = { c, '\0' };               //   A 1-char string version of c.
             ^

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475044_s336572/logs/xmorse-20041125_10.log