gdbsupport: fix path_join crash with -std=c++17 and -D_GLIBCXX_DEBUG

When building GDB with -std=c++17 and -D_GLIBCXX_DEBUG=1, I get:

  $ ./gdb -nx --data-directory=data-directory -q -ex "maint selftest path_join"
  /usr/include/c++/11.2.0/string_view:233: constexpr const value_type& std::basic_string_view<_CharT, _Traits>::operator[](std::basic_string_view<_CharT, _Traits>::size_type) const [with _CharT = char; _Traits = std::char_traits<char>; std::basic_string_view<_CharT, _Traits>::const_reference = const char&; std::basic_string_view<_CharT, _Traits>::size_type = long unsigned int]: Assertion  '__pos < this->_M_len' failed.

The problem is that we're passing an empty string_view to
IS_ABSOLUTE_PATH.  IS_ABSOLUTE_PATH accesses [0] on that string_view,
which is out-of-bounds.

The reason this is not seen with -std less than c++17 is that our local
copy of string_view (used with C++ < 17) does not have the assert in
operator[], as that wouldn't work in a constexpr method:

  5890af36e5/gdbsupport/gdb_string_view.h (L180)

IS_ABSOLUTE_PATH is normally used with null-terminated string.  It's
fine to pass an empty null-terminated string to IS_ABSOLUTE_PATH,
because index 0 in such a string is valid.  But not with an empty
string_view.

Fix that by avoiding the "call" to IS_ABSOLUTE_PATH if the string_view
is empty.

Change-Id: Idf4df961b63f513b3389235e93814c02b89ea32e

gdbsupport/pathstuff.cc

@@ -200,7 +200,7 @@ path_join (gdb::array_view<const gdb::string_view> paths)
       const gdb::string_view path = paths[i];
 
       if (i > 0)
-	gdb_assert (!IS_ABSOLUTE_PATH (path));
+	gdb_assert (path.empty () || !IS_ABSOLUTE_PATH (path));
 
       if (!ret.empty () && !IS_DIR_SEPARATOR (ret.back ()))
 	  ret += '/';