[gdb] Fix sig_write for null gdb_stderr

When running test-case gdb.tui/tui-layout-asm.exp with target board
dwarf5-fission-debug-types, the test-case fails and I get a core dump:
...
 # of unexpected core files      1
...

Looking at the backtrace of the core file, what seems to be happening is that:
- gdbpy_flush attempts to flush gdb_stdout, which is nullptr
- that causes a segfault
- gdb intercepts this and starts to handle it using handle_fatal_signal
- handle_fatal_signal calls sig_write, which attempts to write to gdb_stderr,
  which is nullptr,
- that causes another segfault
- gdb exits

I managed to reproduce the problem by the following trigger patch in
stdin_event_handler:
...
-  if (error)
+  if (1 || error)
     {
       current_ui = main_ui;
       ui->unregister_file_handler ();
-      if (main_ui == ui)
+      if (1 || main_ui == ui)
 	{
 	  gdb_printf (gdb_stderr, _("error detected on stdin\n"));
+	  gdb_stderr = nullptr;
+	  gdb_stdout = nullptr;
+	  gdb_stdlog = nullptr;
 	  quit_command ((char *) 0, 0);
 	}
...
which gives us:
...
$ gdb
(gdb) <q>error detected on stdin
Segmentation fault (core dumped)
$ q
...

Fix sig_write to handle the case that gdb_stderr == nullptr, such that we get
instead:
...
$ gdb
(gdb) <q>error detected on stdin

Fatal signal: Segmentation fault
----- Backtrace -----
  ...
---------------------
A fatal error internal to GDB has been detected, further
debugging is not possible.  GDB will now terminate.

This is a bug, please report it.  For instructions, see:
<https://www.gnu.org/software/gdb/bugs/>.

Segmentation fault (core dumped)
$ q
...

Tested on x86_64-linux.

Approved-By: Simon Marchi <simon.marchi@efficios.com>