internal/ghsa: fix nil pointer dereference

Change-Id: I6123f0d1dc47eb0f5de1c70f0e32d76659d5fe0e
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/581796
Auto-Submit: Tatiana Bradley <tatianabradley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/ghsa/ghsa.go b/internal/ghsa/ghsa.go
index dfb7ba7..ba9ea3c 100644
--- a/internal/ghsa/ghsa.go
+++ b/internal/ghsa/ghsa.go
@@ -106,13 +106,17 @@
 	if sa.Vulnerabilities.PageInfo.HasNextPage {
 		return nil, fmt.Errorf("%s has more than 100 vulns", sa.GhsaID)
 	}
+	var permalink string
+	if sa.Permalink.URL != nil {
+		permalink = sa.Permalink.URL.String()
+	}
 	s := &SecurityAdvisory{
 		ID:          sa.GhsaID,
 		Identifiers: sa.Identifiers,
 		Summary:     sa.Summary,
 		Description: sa.Description,
 		Origin:      sa.Origin,
-		Permalink:   sa.Permalink.URL.String(),
+		Permalink:   permalink,
 		References:  sa.References,
 		PublishedAt: sa.PublishedAt,
 		UpdatedAt:   sa.UpdatedAt,