fix(artifactory): ignore extensions without a manifest

This ignores extensions where the manifest download from
artifactory failed instead of crashing the process with a
segfault.

It fixes #68

Author: janLo

storage/artifactory.go

@@ -405,6 +405,13 @@ func (s *Artifactory) WalkExtensions(ctx context.Context, fn func(manifest *VSIX
 		return err
 	}
 	for _, ext := range extensions {
+		if ext.manifest == nil {
+			id := ExtensionID(ext.publisher, ext.name, ext.versions[0].Version)
+			s.logger.Warn(ctx, "Ignore extension with missing manifest",
+				slog.F("id", id),
+				slog.F("targetPlatform", ext.versions[0].TargetPlatform))
+			continue
+		}
 		if err = fn(ext.manifest, ext.versions); err != nil {
 			return err
 		}