fsapi.c: Fix memory leak caused by incorrect tree insertion assumption.

The function 'refs_rb_tree_insert_node' is documented to return 1 if the
node insertion was successful, however it returns 1 and leaks the
allocated node if the tree is NULL, which is far from a successful
insertion.
This was pointed out in issue #28, which I have never hit myself but
clearly that was just out of luck (or bad luck). Fixed by creating the
tree in 'fsapi_node_cache_enter' if it does not currently exist before
proceeding with the insertion.