Initialize strtok_r's saveptr to NULL

Building gdb with --enable-build-with-cxx=no trips on a warning:

 ../../binutils-gdb/gdb/rust-lang.c:173:15: error: saveptr may be used
 uninitialized in this function [-Werror=maybe-uninitialized]
     ret.name = concat (TYPE_NAME (type), "::", token, (char *) NULL);

The problem is that gcc doesn't understand that "tail" can never be
NULL in the call to strtok_r:

      name = xstrdup (TYPE_FIELD_NAME (type, 0));
      cleanup = make_cleanup (xfree, name);
      tail = name + strlen (RUST_ENUM_PREFIX);
...
      for (token = strtok_r (tail, "$", &saveptr);

Fix this by always initializing saveptr.

2016-06-29  Manish Goregaokar  <manish@mozilla.com>

gdb/ChangeLog:
    * rust-lang.c (rust_get_disr_info): Initialize saveptr to NULL.