MemMapFs `Remove` file panics

[acabarbaye]

I am using 1.9.5 when trying to remove a file such as /tmp/test-973171232/testDir/test-129751122.test using MemMapFS, I am getting a panic panic: parent of /tmp/test-973171232/testDir/test-129751122.test is nil coming from unRegisterWithParent (https://github.com/spf13/afero/blob/d92c300a1b6eaa1690b7aecab2ef7fd0d64a94e1/memmap.go#L72). The same used to be possible in previous versions and therefore, this looks like a regression.