Remove incorrect assertion from filesystem::absolute

The assertion is wrong, it should be *s.end() == 0, but that's not
allowed. Just remove it, but keep the comment.

	* src/c++17/fs_ops.cc (absolute(const path&, error_code&))
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Remove bogus assertion.

From-SVN: r271300

libstdc++-v3/ChangeLog

 2019-05-16  Jonathan Wakely  <jwakely@redhat.com>
 
+	* src/c++17/fs_ops.cc (absolute(const path&, error_code&))
+	[_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Remove bogus assertion.
+
 	* include/std/variant (__overload_set): Remove.
 	(_Arr): New helper.
 	(_Build_FUN): New class template to define a single FUN overload,

libstdc++-v3/src/c++17/fs_ops.cc

@@ -96,6 +96,7 @@ fs::absolute(const path& p, error_code& ec)
     }
 
 #ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
+  // s must remain null-terminated
   wstring_view s = p.native();
 
   if (p.has_root_directory()) // implies !p.has_root_name()
@@ -108,9 +109,6 @@ fs::absolute(const path& p, error_code& ec)
       s.remove_prefix(std::min(s.length(), pos) - 1);
     }
 
-  // s must be null-terminated
-  __glibcxx_assert(!s.empty() && s.back() == 0);
-
   uint32_t len = 1024;
   wstring buf;
   do