NULL-check s2n_cert_chain_and_key_get_pkey_type

This adds a check to guard the case where a certificate chain
is empty. In case it is, S2N_PKEY_TYPE_UNKNOWN is returned.

crypto/s2n_certificate.c

@@ -619,6 +619,11 @@ void *s2n_cert_chain_and_key_get_ctx(struct s2n_cert_chain_and_key *cert_and_key
 
 s2n_pkey_type s2n_cert_chain_and_key_get_pkey_type(struct s2n_cert_chain_and_key *chain_and_key)
 {
+    if (chain_and_key == NULL
+	 || chain_and_key->cert_chain == NULL
+	 || hain_and_key->cert_chain->head == NULL) {
+        return S2N_PKEY_TYPE_UNKNOWN;
+    }
     return chain_and_key->cert_chain->head->pkey_type;
 }