py-imaging, py-pillow -- Buffer overflow in FLI decoding code

The Pillow maintainers report:

In all versions of Pillow, dating back at least to the last PIL
1.1.7 release, FliDecode.c has a buffer overflow error.
There is a memcpy error where x is added to a target buffer
address. X is used in several internal temporary variable roles,
but can take a value up to the width of the image. Im->image[y]
is a set of row pointers to segments of memory that are the size of
the row. At the max y, this will write the contents of the line off
the end of the memory buffer, causing a segfault.
This issue was found by Alyssa Besseling at Atlassian.