Fix memory leaks in DexString wrapper and in unit tests.

This addresses the art-gtest-asan failure from http://r.android.com/867370:
http://ab/5224785

Test: env SANITIZE_HOST=address m test-art-host-gtest-dex_file_supp_test
Bug: 119632407
Change-Id: I84f761561223c7da56a335b6691189020027a4a0
2 files changed