crypt32: Don't dereference an output pointer which may be NULL.

dlls/crypt32/store.c

@@ -899,6 +899,7 @@ BOOL WINAPI CertAddCertificateContextToStore(HCERTSTORE hCertStore,
         if (existing)
         {
             CertContext_CopyProperties(existing, pCertContext);
+            if (ppStoreContext)
                 *ppStoreContext = CertDuplicateCertificateContext(existing);
         }
         else