lopdf has a stack overflow via deeply nested PDF objects

`lopdf::Document::load_mem` (and the other `load*` entry points) parses nested PDF arrays and dictionaries with unbounded recursion. A small crafted PDF whose Catalog contains a deeply nested array (`/X [[[ … ]]]`, on the order of 10,000 levels) exhausts the call stack and aborts the process with `SIGABRT`. Because this is a stack-overflow abort rather than a `panic!`, it cannot be caught with `catch_unwind`: any service that parses untrusted PDF input with lopdf can be crashed by a ~21 KB file, resulting in a denial of service. Confirmed on lopdf 0.41.0 and earlier; fixed in 0.42.0. Default configuration, no features changed. ## Proof of concept ```rust fn main() { let bytes = std::fs::read("poc.pdf").unwrap(); // ~10,380-deep nested array in the Catalog let _ = lopdf::Document::load_mem(&bytes); // stack overflow -> SIGABRT } ``` An equivalent PoC is a minimal PDF whose Catalog `/X` value is `"[" * 10380 + "]" * 10380`. ## Suggested fix Enforce a maximum object-nesting depth in the parser and return an `Err` instead of recursing without bound.