Fix printing of global variable stubs if no inferior is running

Since 3c45e9f915 gdb crashes when trying
to print a global variable stub without a running inferior, because of
a missing nullptr-check (the block_scope function took care of that
check before it was converted to a method).

With this check it works again:
```
(gdb) print s
$1 = <incomplete type>
```

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31128
Approved-By: Tom Tromey <tom@tromey.com>
(cherry picked from commit 576745e26c)

gdb/cp-namespace.c

@@ -1026,7 +1026,11 @@ cp_lookup_transparent_type (const char *name)
 
   /* If that doesn't work and we're within a namespace, look there
      instead.  */
-  scope = get_selected_block (0)->scope ();
+  const block *block = get_selected_block (0);
+  if (block == nullptr)
+    return nullptr;
+
+  scope = block->scope ();
 
   if (scope[0] == '\0')
     return NULL;