[BUG] jffs2: possible null-pointer dereference in jffs2_scan_dirty_space()

[Li Tuo]

Hello,

Our static analysis tool finds a possible null-pointer dereference in 
the jffs2 driver in Linux 5.14.0-rc3:

The variable jeb->last_node is checked in:
673:    if (jeb->last_node && ref_obsolete(jeb->last_node))

This indicates that jeb->last_node can be NULL.
If so, the function jffs2_link_node_ref() is called with the argument jeb:
685:    jffs2_link_node_ref(c, jeb, ofs, size, NULL);

In the called function, jeb->last_node is assigned to the pointer ref, 
which means ref is also NULL.
However, the possible NULL pointer ref is dereferenced in some 
statements such as:
597:    dbg_noderef("Last node at %p is (%08x,%p)\n", ref, 
ref->flash_offset, ref->next_in_ino);
600:    while (ref->flash_offset != REF_EMPTY_NODE)

I am not quite sure whether this possible null-pointer dereference is 
real and how to fix it if it is real.
Any feedback would be appreciated, thanks!

Reported-by: TOTE Robot <oslab at tsinghua.edu.cn>

Best wishes,
Tuo Li