Fix some malloc test failures.

These only affect the tests.

Change-Id: If22d047dc98023501c771787b485276ece92d4a2
Reviewed-on: https://boringssl-review.googlesource.com/7573
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/x509/x509_test.cc

diff --git a/crypto/x509/x509_test.cc b/crypto/x509/x509_test.cc
index 1fce619..650163a 100644
--- a/crypto/x509/x509_test.cc
+++ b/crypto/x509/x509_test.cc
@@ -243,6 +243,9 @@
 // bumping the reference counts for each certificate in question.
 static STACK_OF(X509)* CertsToStack(const std::vector<X509*> &certs) {
   ScopedX509Stack stack(sk_X509_new_null());
+  if (!stack) {
+    return nullptr;
+  }
   for (auto cert : certs) {
     if (!sk_X509_push(stack.get(), cert)) {
       return nullptr;