Fix crash on malformed nested brace/paren patterns

[afonsojanu]

Fixes #60.

braces('{{*(()', { expand: true })
// TypeError: Cannot read properties of undefined (reading 'push')

The crash is in walk(). When it hits a leaf node with a value, it climbs the node's parent chain looking for the nearest ancestor typed brace or root, then pushes onto that ancestor's queue array:

while (block.type !== 'brace' && block.type !== 'root' && block.parent) {
  block = block.parent;
  queue = block.queue;
}
...
queue.push(append(queue.pop(), child.value));

That assumes the ancestor it lands on already has queue set, which normally happens because walk() sets node.queue = [] right when it's called on a node. For this particular pattern, the parser builds a paren node whose parent chain runs through a brace node that walk() never actually visits on its own, since the parser leaves it out of the tree it hands back for input this malformed. So that ancestor's queue stays undefined, and pushing onto it blows up.

I traced this with some throwaway logging rather than guessing: for {{*((), the crash happens exactly when reaching the inner paren node's () value, at which point block.type is 'brace' and block.queue is undefined.

The fix just falls back to an empty array in both places this climb happens, when it lands on a node without a queue. This matches how the rest of this function already treats other malformed patterns (falling back to something reasonable rather than throwing), so {{*(() now expands to ['{{*('] instead of crashing.

Added a test in test/regression.js alongside the existing malformed-pattern cases there, covering the reported pattern plus two related shapes ({{{ and ((()). Confirmed it fails on master and passes with the fix. Ran the full suite locally, 895 passing.