NativeCrypto: return of 0 is error for EVP_Sign/VerifyFinal

We need to check the ERR stack on a return code of 0. Previously there
was a comment indicating the weird behavior about DSA keys throwing
after a check for a return value of -1, but this API is never supposed
to return anything other than 1 for success or 0 for failure.

Bug: 18869265
Change-Id: Ic871c63b6d65949053819950ed8053f47501bd60
2 files changed