Fixing syzkaller bugs

[Manas Ghandat <ghandatmanas@xxxxxxxxx>]

I was looking at the following bug 
https://syzkaller.appspot.com/bug?extid=5184326923f180b9d11a . Adding a 
null check at line 2556 of fix_node.c should fix this issue but i am 
unable to understand on how to call reiserfs_panic.