NULL dereference validating DSA public key

An invalid pointer dereference on read can be triggered when an application tries to check a malformed DSA public key by the EVP_PKEY_public_check() function. This will most likely lead to an application crash. This function can be called on public keys supplied from untrusted sources which could allow an attacker to cause a denial of service attack. The TLS implementation in OpenSSL does not call this function but applications might call the function if there are additional security requirements imposed by standards such as FIPS 140-3.