lib: fix gcc warning in printf call

[vszakats]

Do not pass NULL to printf %s.

Seen with gcc 13.2.0 on Debian:

.../curl/lib/connect.c:696:27: warning: '%s' directive argument is null [-Wformat-overflow=]

Ref: https://github.com/curl/curl-for-win/actions/runs/6476161689/job/17584426483#step:3:11104

Ref: #10284
Co-authored-by: Jay Satiro
Closes #12082

---

In function 'is_connected',
    inlined from 'cf_he_connect' at /home/runner/work/curl-for-win/curl-for-win/curl/lib/connect.c:906:16:
/home/runner/work/curl-for-win/curl-for-win/curl/lib/connect.c:696:27: warning: '%s' directive argument is null [-Wformat-overflow=]
  696 |     CURL_TRC_CF(data, cf, "%s assess started=%d, result=%d",
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/runner/work/curl-for-win/curl-for-win/curl/lib/curl_trc.h:123:38: note: in definition of macro 'CURL_TRC_CF'
  123 |          Curl_trc_cf_infof(data, cf, __VA_ARGS__); } while(0)
      |                                      ^~~~~~~~~~~
In file included from /home/runner/work/curl-for-win/curl-for-win/curl/bld/lib/CMakeFiles/libcurl_object.dir/Unity/unity_0_c.c:49:
/home/runner/work/curl-for-win/curl-for-win/curl/lib/connect.c: In function 'cf_he_connect':
/home/runner/work/curl-for-win/curl-for-win/curl/lib/connect.c:696:28: note: format string is defined here
  696 |     CURL_TRC_CF(data, cf, "%s assess started=%d, result=%d",
      |                            ^~
make[2]: Leaving directory '/home/runner/work/curl-for-win/curl-for-win/curl/bld'

[jay]

Since the warning text is not in this one I would add a brief explanation of it or the action taken, example

lib: fix gcc warning

- Do not pass NULL to printf %s.

...

The problem with the warnings in some cases like this one is they are confusing and take extra cognitive processing so that's why I was suggesting a summary of sorts so it's easy to see what the warning is actually for or what is being fixed. I agree it is good to keep the actual warning in the PR so we have a record of what it actually said. The CI links are all ephemeral so in the commit message they are only good for so long.