Properly validate multi-URL foreign layers

The existing code effectively ignored errors from all but the last of a
foreign layer's URLs.

Signed-off-by: Noah Treuhaft <noah.treuhaft@docker.com>

registry/storage/schema2manifesthandler_test.go

@@ -78,6 +78,11 @@ func TestVerifyManifestForeignLayer(t *testing.T) {
 			[]string{"https://foo/bar", ""},
 			errInvalidURL,
 		},
+		{
+			foreignLayer,
+			[]string{"", "https://foo/bar"},
+			errInvalidURL,
+		},
 		{
 			foreignLayer,
 			[]string{"http://foo/bar"},