libstdc++: Fix test failure due to -Wnonnull warnings

This test fails in the Fedora RPM build (but not elsewhere, for unknown
reasons). The warning is correct, we're passing a null pointer.

	* testsuite/tr1/8_c_compatibility/cstdlib/functions.cc: Do not pass
	a null pointer to functions with nonnull(1) attribute.

libstdc++-v3/ChangeLog

+2020-03-12  Jonathan Wakely  <jwakely@redhat.com>
+
+	* testsuite/tr1/8_c_compatibility/cstdlib/functions.cc: Do not pass
+	a null pointer to functions with nonnull(1) attribute.
+
 2020-03-11  Patrick Palka  <ppalka@redhat.com>
 
 	* include/std/ranges (split_view::_OuterIter::_OuterIter): Typo fix,

libstdc++-v3/testsuite/tr1/8_c_compatibility/cstdlib/functions.cc

@@ -30,7 +30,7 @@ void test01()
 #if _GLIBCXX_USE_C99_STDLIB
 
   long long i = 0;
-  const char* s = 0;
+  const char* s = "";
   char** endptr = 0;
   int base = 0;