Fix: Avoid a false type assertion in the inmemory driver

This issue was discovered by the following fuzzer:
https://github.com/cncf/cncf-fuzzing/blob/main/projects/distribution/inmemory_fuzzer.go#L24

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

registry/storage/driver/inmemory/mfs.go

@@ -244,11 +244,16 @@ func (d *dir) delete(p string) error {
 		return errNotExists
 	}
 
-	if _, ok := parent.(*dir).children[filename]; !ok {
+	parentDir, ok := parent.(*dir)
+	if !ok {
+		return errIsNotDir
+	}
+
+	if _, ok := parentDir.children[filename]; !ok {
 		return errNotExists
 	}
 
-	delete(parent.(*dir).children, filename)
+	delete(parentDir.children, filename)
 	return nil
 }