libstdc++: Fix tr1/8_c_compatibility/cstdio/functions.cc regression with recent glibc

The following testcase started FAILing recently after the
https://sourceware.org/git/?p=glibc.git;a=commit;h=64b1a44183a3094672ed304532bedb9acc707554
glibc change which marked vfscanf with nonnull (1) attribute.
While vfwscanf hasn't been marked similarly (strangely), the patch changes
that too.  By using va_arg one hides the value of it from the compiler
(volatile keyword would do too, or making the FILE* stream a function
argument, but then it might need to be guarded by #if or something).

2023-10-13  Jakub Jelinek  <jakub@redhat.com>

	* testsuite/tr1/8_c_compatibility/cstdio/functions.cc (test01):
	Initialize stream to va_arg(ap, FILE*) rather than 0.
	* testsuite/tr1/8_c_compatibility/cwchar/functions.cc (test01):
	Likewise.

(cherry picked from commit badb798f)