show: Don't crash if thread_info is absent in core

This is how zombies live.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-show.c

@@ -430,6 +430,9 @@ static void show_core_regs(UserX86RegsEntry *regs)
 
 void show_thread_info(ThreadInfoX86 *thread_info)
 {
+	if (!thread_info)
+		return;
+
 	pr_msg("\t---[ Thread info ]---\n");
 	pr_msg("\tclear_tid_addr:  0x%lx\n", thread_info->clear_tid_addr);
 	pr_msg("\n");