[Rust] Don't leak the callbacks within DownloadInstance::perform_custom_request

Author: bdash

rust/src/download_provider.rs

@@ -253,6 +253,7 @@ impl DownloadInstance {
             )
         };
 
+        unsafe { drop(Box::from_raw(callbacks)) };
         if result < 0 {
             unsafe { BNFreeDownloadInstanceResponse(response) };
             return Err(self.get_error());