Avoid crashes in 'display_count_lines' when current buffer was killed

* src/xdisp.c (Fformat_mode_line):
* src/fns.c (Fline_number_at_pos): Don't allow to count lines in a
dead buffer.  (Bug#65060)

src/fns.c

@@ -6123,6 +6123,9 @@ from the absolute start of the buffer, disregarding the narrowing.  */)
 {
   ptrdiff_t pos_byte, start_byte = BEGV_BYTE;
 
+  if (!BUFFER_LIVE_P (current_buffer))
+    error ("Attempt to count lines in a dead buffer");
+
   if (MARKERP (position))
     {
       /* We don't trust the byte position if the marker's buffer is

src/xdisp.c

@@ -27653,6 +27653,8 @@ are the selected window and the WINDOW's buffer).  */)
   if (NILP (buffer))
     buffer = w->contents;
   CHECK_BUFFER (buffer);
+  if (!BUFFER_LIVE_P (XBUFFER (buffer)))
+    error ("Attempt to format a mode line for a dead buffer");
 
   /* Make formatting the modeline a non-op when noninteractive, otherwise
      there will be problems later caused by a partially initialized frame.  */