dec_term: avoid integer overflow for LIST_EXT
dec_term: avoid integer overflow for LIST_EXT

- use unsigned type for the length n
- perform mutiplication by 2 in wider type

Fixes #6439

bjorng/otp,vinoski/otp,erlang/otp,vinoski/otp,emacsmirror/erlang,erlang/otp,vinoski/otp,erlang/otp,vinoski/otp,bjorng/otp,vinoski/otp,bjorng/otp,emacsmirror/erlang,emacsmirror/erlang,bjorng/otp,erlang/otp,bjorng/otp,erlang/otp,emacsmirror/erlang,vinoski/otp,erlang/otp,emacsmirror/erlang,emacsmirror/erlang,bjorng/otp,erlang/otp,erlang/otp,erlang/otp,erlang/otp,bjorng/otp,vinoski/otp,bjorng/otp,emacsmirror/erlang,bjorng/otp,emacsmirror/erlang,vinoski/otp,vinoski/otp,vinoski/otp,bjorng/otp,emacsmirror/erlang,emacsmirror/erlang