libext2fs: fix crash in ext2fs_image_super_write() on Big Endian systems

libext2fs: fix crash in ext2fs_image_super_write() on Big Endian systems

This is a similar fix as c9a8c53b17cc ("libext2fs: fix crash in
ext2fs_open2() on Big Endian systems").

Commit e6069a05: ("Teach ext2fs_open2() to honor the
EXT2_FLAG_SUPER_ONLY flag") changed how the function
ext2fs_group_desc() handled a request for a gdp pointer for a group
larger than the number of groups in the file system; it now returns
NULL, instead of returning a pointer beyond the end of the array.

Previously, the ext2fs_imager_super_write() function would swap all of
the block group descriptors in a block, even if they are beyond the
end of the file system.  This was OK, since we were not overrunning
the allocated memory, since it was rounded to a block boundary.  But
now that ext2fs_group_desc() would return NULL for those gdp, it would
cause ext2fs_open2(), when it was byte swapping the block group
descriptors on Big Endian systems, to dereference a null pointer and
crash.

This commit adds a NULL pointer check to avoid byte swapping those
block group descriptors in a bg descriptor block, but which are beyond
the end of the file system, to address this crash.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Anatoly Pugachev <matorola@gmail.com>