Explicitly initialise two more static pointers

"dtcm: Coverity 88006" fixed this for buf, but not for string in the
same function (and we may as well initialise string_head for consistency
too).

cde/programs/dtcm/dtcm/misc.c

@@ -621,8 +621,8 @@ cm_mbstrlen(char *s) {
 */
 extern char *
 cm_mbchar(char *str) {
-     static char *string;
-     static char *string_head;
+     static char *string = NULL;
+     static char *string_head = NULL;
      static char *buf = NULL;
      int num_byte = 0;