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>

1 files changed, 3 insertions, 0 deletions

diff --git a/gdb/frame.c b/gdb/frame.c
index fae89cb..72a34fc 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -3121,6 +3121,9 @@ frame_info_ptr
 frame_follow_static_link (frame_info_ptr frame)
 {
   const block *frame_block = get_frame_block (frame, nullptr);
+  if (frame_block == nullptr)
+    return {};
+
   frame_block = frame_block->function_block ();
 
   const struct dynamic_prop *static_link = frame_block->static_link ();