Fixed unchecked key blob access in attestKey

In attestKey we did not check a return value after retrieving a key
blob. This results in a segfault if an attestation is requested on a non
existent or corrupted key.

Bug: 135907940
Bug: 138146601
Change-Id: I1e84d334bb93ac878b63e15a59c35631f9e6e397
Merged-In: I1e84d334bb93ac878b63e15a59c35631f9e6e397
1 file changed