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.

(cherry picked from commit 4b62522c9d7d5cb4177d337b9a162df9471f4aff)

Test: Updated dex2oat_test#RunUnquickenMultiDex.
Test: m test-art-host-gtest
Bug: 155366778
Merged-In: Ie369a8822d08be731b8d272fa8556c3939aeb66f
Change-Id: I8f7a0d06dbb9cedd319b9c0d7c340107ce88e56c
2 files changed