Fix a coverity NULL-pointer deref warning in the tortls tests.

Also, make our cert validation code more NULL-resistant.

This is CID 1327891.

src/common/tortls.c

@@ -884,7 +884,7 @@ tor_tls_cert_is_valid(int severity,
   EVP_PKEY *cert_key;
   int r, key_ok = 0;
 
-  if (!signing_cert)
+  if (!signing_cert || !cert)
     goto bad;
 
   EVP_PKEY *signing_key = X509_get_pubkey(signing_cert->cert);

src/test/test_tortls.c

@@ -2680,7 +2680,12 @@ test_tortls_cert_is_valid(void *ignored)
   scert = tor_malloc_zero(sizeof(tor_x509_cert_t));
   ret = tor_tls_cert_is_valid(LOG_WARN, cert, scert, 0);
   tt_int_op(ret, OP_EQ, 0);
+
+  cert = tor_malloc_zero(sizeof(tor_x509_cert_t));
+  ret = tor_tls_cert_is_valid(LOG_WARN, cert, scert, 0);
+  tt_int_op(ret, OP_EQ, 0);
   tor_free(scert);
+  tor_free(cert);
 
   cert = tor_x509_cert_new(read_cert_from(validCertString));
   scert = tor_x509_cert_new(read_cert_from(caCertString));