Fix build warning on Lenny about strtok_r unit test

This fixes a warning in efb8a09f, where Debain Lenny's GCC doesn't get
that
    for (i=0; i<3; ++i) {
      const char *p;
      switch(i) {
       case 0:
         p="X"; break;
       case 1:
         p="Y"; break;
       case 2:
         p="Z"; break;
      }
      printf("%s\n", p);
    }
will never try to print an uninitialezed value.

Found by buildbots.  Bug in no released versions of Tor.