tree.c: avoid theoretically dereferencing NULL

[gperciva]

This diverges from libarchive in a specific and limited way.

There is no such sanity check in modern libarchive, so I doubt that the code can ever reach tree_pop() with (t->stack == NULL). But that's not obvious from the code, so we might as well be safe.

[cperciva]

Looks to me like all code paths leading to tree_pop have already dereferenced t->stack, so this is just a case of "telling the compiler what it should already be able to deduce".

[gperciva]

ok.