scan-build thinks there can be a `NULL` pointer dereference in `while
((i = N_NODES (node->left)) != pos)`, if `node` is `NULL`.

`node` cannot be `NULL`, though, assuming the `n_nodes` member of each
node in the tree is an accurate count of the number of nodes beneath
that point. It controls the tree descent and avoids trying to descend
beneath a leaf.

A static analyser can’t know this though, so let’s add an assertion to
help.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1767