Correctly handle the return value from EVP_Cipher() in the CMAC code

Correctly handle the return value from EVP_Cipher() in the CMAC code

EVP_Cipher() is a very low level routine that directly calls the
underlying cipher function. It's return value semantics are very odd.
Depending on the type of cipher 0 or -1 is returned on error. We should
just check for <=0 for a failure.

Fixes #11957

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11972)