log/windows: Guard against null pointer dereference

This is semantically correct since we can't claim we own a console
before we find out if we have one or need to set one up.

Nobody should be calling using_own_console() before
early_log_file_setup() finishes, but you never know. People in this
project have surprised me before.

src/log_windows.cpp

@@ -484,7 +484,7 @@ void enable_native_console_output()
 
 bool using_own_console()
 {
-	return lfm->owns_console();
+	return lfm && lfm->owns_console();
 }
 
 void finish_log_file_setup()