fix: #39 from_bytes_separate 检查 RustCallStatus 返回值

分配失败时将 out 参数置零并 return，避免返回未初始化栈垃圾。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/zx_document_ffi/src/lib.rs

@@ -157,6 +157,15 @@ pub extern "C" fn ffi_zx_document_ffi_rustbuffer_from_bytes_separate(
             &mut call_status,
         )
     };
+    // Check if allocation succeeded
+    if call_status.code != uniffi::RustCallStatusCode::Success {
+        unsafe {
+            *out_capacity = 0;
+            *out_len = 0;
+            *out_data = std::ptr::null_mut();
+        }
+        return;
+    }
     unsafe {
         *out_capacity = buf.capacity() as u64;
         *out_len = buf.len() as u64;