libstdc++: Fix undefined FILE* operations in test

We only need to check that the constructor doesn't clear errno, so
there's no need to use an invalid FILE* for that.

libstdc++-v3/ChangeLog:

	PR libstdc++/98001
	* testsuite/ext/stdio_filebuf/char/79820.cc: Do not pass invalid
	FILE* to constructor.

libstdc++-v3/testsuite/ext/stdio_filebuf/char/79820.cc

@@ -26,10 +26,11 @@ void
 test01()
 {
   FILE* f = std::fopen("79820.txt", "w");
-  std::fclose(f);
   errno = 127;
   __gnu_cxx::stdio_filebuf<char> b(f, std::ios::out, BUFSIZ);
   VERIFY(errno == 127); // PR libstdc++/79820
+  b.close();
+  std::fclose(f);
 }
 
 int