Fix seg fault in TS_RESP_verify_response()

The TS_RESP_verify_response() function is used for verifying the response
from a TSA. You can set the provided TS_VERIFY_CTX with different flags
depending on what aspects of the response you wish to verify.

A seg fault will occur if you supply the TS_VFY_SIGNER or TS_VFY_TSA_NAME
flags without also specifying TS_VFY_SIGNATURE.

Reviewed-by: Rich Salz <rsalz@openssl.org>