mount_image.c: avoid UBSAN warning in wimfs_listxattr()

mount_image.c: avoid UBSAN warning in wimfs_listxattr()

When 'list == NULL && size == 0', the statement 'end = list + size'
executes 'NULL + 0'.  clang's UndefinedBehaviorSanitizer complains that
this is undefined:

    src/mount_image.c:1518:19: runtime error: applying zero offset to null pointer

This is questionable, but let's avoid it...