c++: Don't call cpp_translate_string on NULL string [PR123918]

My P2246R1 patch caused diagnostics reported by running ubsan
instrumented compiler on cpp26/static_assert1.C - if len is 0,
we don't bother to allocate msg, so it stays NULL, and when I've added
cpp_translate_string call, that can invoke memcpy (something, NULL, 0);
in that case.
While that is no longer UB in C2Y since N3322, libsanitizer doesn't
know that yet and reports it anyway.
While we could just do
      if (len)
        {
...
        }
      else
        msg = "";
there is really no point in trying to translate "" and allocate memory
for that, so the following patch instead by passes that translation for
len == 0.

2026-02-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123918
	* semantics.cc (cexpr_str::extract): Bypass cpp_translate_string
	for len == 0.

0 files changed, 0 insertions, 0 deletions