Fix filesystem test that fails in debug mode

	* testsuite/experimental/filesystem/path/construct/range.cc: Don't
	use basic_string::front() when string might be empty.

From-SVN: r243438

libstdc++-v3/ChangeLog

 2016-12-08  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/experimental/filesystem/path/construct/range.cc: Don't
+	use basic_string::front() when string might be empty.
+
 	* include/debug/array (swap): Add deleted overload.
 	* include/bits/stl_pair.h (swap): Remove redundant inline keyword
 	from deleted overload.

libstdc++-v3/testsuite/experimental/filesystem/path/construct/range.cc

@@ -59,13 +59,14 @@ test01()
     using __gnu_test::test_container;
     using __gnu_test::input_iterator_wrapper;
     // Test with input iterators and const value_types
+
     test_container<char, input_iterator_wrapper>
-      r1(&s.front(), &s.front() + s.size());
+      r1((char*)s.c_str(), (char*)s.c_str() + s.size());
     path p9(r1.begin(), r1.end());
     compare_paths(p1, p9);
 
     test_container<char, input_iterator_wrapper>
-      r2(&s.front(), &s.front() + s.size() + 1); // includes null-terminator
+      r2((char*)s.c_str(), (char*)s.c_str() + s.size() + 1); // includes null-terminator
     path p10(r2.begin());
     compare_paths(p1, p10);
 
@@ -82,12 +83,12 @@ test01()
 #if _GLIBCXX_USE_WCHAR_T
     // Test with input iterators and const value_types
     test_container<wchar_t, input_iterator_wrapper>
-      r5(&ws.front(), &ws.front() + ws.size());
+      r5((wchar_t*)ws.c_str(), (wchar_t*)ws.c_str() + ws.size());
     path p13(r5.begin(), r5.end());
     compare_paths(p1, p13);
 
     test_container<wchar_t, input_iterator_wrapper>
-      r6(&ws.front(), &ws.front() + ws.size() + 1); // includes null-terminator
+      r6((wchar_t*)ws.c_str(), (wchar_t*)ws.c_str() + ws.size() + 1); // includes null-terminator
     path p14(r6.begin());
     compare_paths(p1, p14);