Resolves #3496

In that issue, a bus error occurs when opening an image and saving it back to the same location. It was described as relating to mmap. Investigating, I find that the bus error occurs at

So presumably that is conflicting with mmap.

The same block of code that sets up mmap also marks the image as readonly.

https://pillow.readthedocs.io/en/5.4.1/releasenotes/3.3.2.html

The image was marked readonly, so Pillow would not ordinarily write to that memory without duplicating the image first.

Given the general testing requirement of operating on temp files, and since the test involves opening and saving back the same file, the test can't be run on Windows, as temp files can't be opened twice on that operating system - https://docs.python.org/2/library/tempfile.html

Whether the name can be used to open the file a second time, while the named temporary file is still open, varies across platforms (it can be so used on Unix; it cannot on Windows NT or later).