Fix crash when calling Frame.static_link

Fix crash when calling Frame.static_link

If you try to call Frame.static_link for a frame without debug info,
gdb crashes:
```
Temporary breakpoint 1, 0x000000013f821650 in main ()
(gdb) py print(gdb.selected_frame().static_link())

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
```

The problem was a missing check if get_frame_block returns nullptr
inside frame_follow_static_link.

With this, it works:
```
Temporary breakpoint 1, 0x000000013f941650 in main ()
(gdb) py print(gdb.selected_frame().static_link())
None
```

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31366
Approved-By: Tom Tromey <tom@tromey.com>