OLD TESTS: Fix use after free in unsolicited_[non]immediate_data tests.

After destroying iscsi, nullify iscsi_url's reference to it to prevent a use
after free.

Detected by clang's AddressSanitizer.
Merge pull request #87 from sitsofe/oldtests

OLD TESTS: Fix use after free in unsolicited_[non]immediate_data tests.