Fix heap overflow with UTF-8 meta characters (#206)

If a meta character has its high bit set, then the %x format would
interpret the character as a signed integer on most systems, i.e.
on systems which have signed chars instead of unsigned chars.

Cast *s to guarantee that only two hex digits are printed, otherwise
the allocated space would be not enough.

How to reproduce (-fsanitize=address, HAVE_POPEN setup, e.g. Linux):

LESSMETACHARS=$(echo -e '\xff') less -f /dev/null
(now type "-T x" and press enter to trigger lglob function call)

filename.c

@@ -760,7 +760,7 @@ lglob(filename)
  SNPRINTF4(cmd, len, "%s -p0x%x -d0x%x -e%s ", lessecho, openquote, closequote, esc);
  free(esc);
  for (s = metachars(); *s != '\0'; s++)
- sprintf(cmd + strlen(cmd), "-n0x%x ", *s);
+ sprintf(cmd + strlen(cmd), "-n0x%x ", (unsigned char) *s);
  sprintf(cmd + strlen(cmd), "-- %s", filename);
  fd = shellcmd(cmd);
  free(cmd);