Do not allow negative offset with memory mapping

[radarhere]

Before #9046 allowed memory mapping to be used with raw_negative_stride.bin, the negative offset in the tile triggered

Traceback (most recent call last):
  File "PIL/ImageFile.py", line 341, in load
    seek(offset)
OSError: [Errno 22] Invalid argument

Now however, it doesn't. This PR raises a specific error when a negative offset is used with memory mapping.