platform.libc_ver() raises TypeError on a threaded libc without a version

Bug report

platform.libc_ver() raises TypeError when it scans a binary that references
a threaded C library (such as libc_r) with no version number:

>>> platform.libc_ver(executable=some_binary_referencing_libc_r_so)
Traceback (most recent call last):
  ...
TypeError: 'NoneType' object is not subscriptable

In the so branch of the scanning loop, when a match has a thread suffix
(_r/_pthread) but no version, ver is still None and gets subscripted.
This is a regression from gh-90548 (PR #131313), so 3.14 and later are affected.

CPython versions tested on:

3.16 (main)

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-155455

从扫描循环中的 platform.libc_ver() 及其 so 分支开始，然后在进行更改之前查看相关的 PR gh-155455。使用一个引用了无版本线程化 C 库的二进制文件重现该情况，并在调用不再引发所报告的 TypeError 时认为问题已解决。