icu: fix NULL dereference in `unum_clone`

If `dynamic_cast` fails, then NULL is returned. Even thought
assertion is set, we cannot rely on it, as we don't use debug
version of icu. Let's check if `rbnf` variable is not NULL
explicitly.

If it somehow turned out to be NULL, then memory allocation
error will be thrown.

Closes tarantool/security#61

NO_CHANGELOG=<security fix>
NO_DOC=<security fix>
NO_TEST=<third-party security fix>

(cherry picked from commit 62bb71cf)