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>

2 files changed, 17 insertions, 0 deletions

diff --git a/gdb/frame.c b/gdb/frame.c
index 4a46ccb..87fb3d7 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -3127,6 +3127,9 @@ 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 ();
diff --git a/gdb/testsuite/gdb.python/py-frame.exp b/gdb/testsuite/gdb.python/py-frame.exp
index 16177c8..ddb1829 100644
--- a/gdb/testsuite/gdb.python/py-frame.exp
+++ b/gdb/testsuite/gdb.python/py-frame.exp
@@ -179,3 +179,17 @@ gdb_test "python print(gdb.selected_frame().language())" "c" \
 gdb_test "python print(gdb.selected_frame().read_register(list()))" \
     ".*Invalid type for register.*" \
     "test Frame.read_register with list"
+
+# Compile again without debug info.
+gdb_exit
+if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} {}] } {
+    return -1
+}
+
+if {![runto_main]} {
+    return 0
+}
+
+# Test if Frame.static_link works for a frame without debug info.
+gdb_test "python print(gdb.selected_frame().static_link())" "None" \
+    "test Frame.static_link for a frame without debug info"