KeyMint VTS: don't crash on invalid patchlevel

If vendor/boot patchlevel is shorter than the expected YYYYMMDD format,
fail properly rather than crashing the VTS test process.

Bug: 201946955
Test: VtsAidlKeyMintTargetTest
Change-Id: Icf3541e1b76675871672edec8590ec1821770acf
1 file changed