icu: fix NULL dereference in `enumEitherTrie`

According to the business logic and assertions `idx` and `data32`
variables cannot be equal to NULL at the same time. However, we
cannot rely on assertions.

Let's check that explicitly. If this situation occurs somehow
the function exits as we cannot recover from this situation: we
don't have sources, from which values for enumeration can be taken.
Moreover, continuing of the code execution is such situation may
lead to accessing NULL if `c<limit`.

Closes tarantool/security#59

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

(cherry picked from commit 73b01ea5)