Add asserts in testing to fix Coverity alert (#1864)

### Description of changes: 
Adds two new asserts to a test to prevent dereferencing a NULL ptr. This
was called out by a Coverity alert.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

crypto/pem/pem_test.cc

@@ -393,6 +393,7 @@ TEST(PEMTest, WriteReadTraditionalPem) {
   ASSERT_TRUE(pkey_read);
 
   EC_KEY *pkey_eckey = EVP_PKEY_get0_EC_KEY(pkey.get());
+  ASSERT_TRUE(pkey_eckey);
   const BIGNUM *orig_priv_key = EC_KEY_get0_private_key(ec_key.get());
   const BIGNUM *read_priv_key = EC_KEY_get0_private_key(pkey_eckey);
   ASSERT_EQ(0, BN_cmp(orig_priv_key, read_priv_key));
@@ -419,6 +420,7 @@ TEST(PEMTest, WriteReadTraditionalPem) {
   ASSERT_TRUE(pkey_read);
 
   RSA *pkey_rsa = EVP_PKEY_get0_RSA(pkey.get());
+  ASSERT_TRUE(pkey_rsa);
   EXPECT_EQ(0, BN_cmp(RSA_get0_d(pkey_rsa), RSA_get0_d(rsa.get())));
   EXPECT_EQ(0, BN_cmp(RSA_get0_d(pkey_rsa), RSA_get0_d(rsa.get())));