memtx: fix potential NULL dereference in memtx_space_ephemeral_delete

Now, delete in ephemeral space is obviously incorrect - if we try to
delete a tuple, which is not present in index, NULL dereference will
happen. Fortunately, ephemeral spaces are used for internal purposes
only, so, most likely, this never happens. Let's fix this part not to
confuse code analyzers.

Closes https://github.com/tarantool/security/issues/38

NO_TEST=shouldn't normally happen
NO_CHANGELOG=shouldn't normally happen
NO_DOC=shouldn't normally happen