Problem: nvim_parse_expression null pointer dereference with unmatched ]
followed by a node.
Solution: if ast_stack was empty, set new_top_node_p to top of the stack after
pushing the list literal node; similar to what's done for curlies.
This bug was originally found by a Matrix user, but I couldn't remember how to
trigger it... Ran into the other crash while finding a repro. :P
a5e5ec8910