squashfs: squashfs_read_data need to check if the length is 0

When the length passed in is 0, the pagemap_scan_test_walk() caller should
bail.  This error causes at least a WARN_ON().

Link: https://lkml.kernel.org/r/20231116031352.40853-1-lizhi.xu@windriver.com


Reported-by:  <syzbot+32d3767580a1ea339a81@syzkaller.appspotmail.com>
Closes: https://lkml.kernel.org/r/0000000000000526f2060a30a085@google.com


Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Reviewed-by: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>