ANDROID: dm: android-verity: Verify header before fetching table

Move header validation logic before reading the verity_table as
an invalid header implies the table is invalid as well.

(Cherry-picked from:
https://partner-android-review.git.corp.google.com/#/c/625203

)

BUG: 29940612
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: Ib34d25c0854202f3e70df0a6d0ef1d96f0250c8e