Verify: check if summary is nil before accessing cert. (#43)

Before, if we returned (nil, err), this would panic. This change adds an
extra check to make sure that summary.Cert can be safely accessed.

Signed-off-by: Billy Lynch <billy@chainguard.dev>

command_verify.go

@@ -125,7 +125,7 @@ func verifyDetached() error {
 
 	summary, err := git.Verify(context.Background(), rekor, buf.Bytes(), sig.Bytes())
 	if err != nil {
-		if summary.Cert != nil {
+		if summary != nil && summary.Cert != nil {
 			emitBadSig(summary.Cert)
 		} else {
 			// TODO: We're omitting a bunch of arguments here.