dex2oat: Fix vdex file invalidation.

We erroneously stored the invalid vdex magic "wdex" after
the header because we already read the header and did not
seek back to the start of the file.

Test: Updated dex2oat_test#RunUnquickenMultiDex.
Test: m test-art-host-gtest
Bug: 155366778
Change-Id: Ie369a8822d08be731b8d272fa8556c3939aeb66f
2 files changed