gdbsupport: allow passing nullptr to checked_static_cast

Both static_cast and dynamic_cast handle nullptr (they return nullptr),
so I think checked_static_cast should too.  This will allow doing a null
check after a checked_static_cast:

  cooked_index_vector *table
    = (gdb::checked_static_cast<cooked_index_vector *>
       (per_bfd->index_table.get ()));
  if (table != nullptr)
    return;

Change-Id: If5c3134e63696f8e417c87b5f3901240c9f7ea97

gdbsupport/gdb-checked-static-cast.h

@@ -54,6 +54,9 @@ checked_static_cast (V *v)
 		 "types must be related");
 
 #ifdef DEVELOPMENT
+  if (v == nullptr)
+    return nullptr;
+
   T result = dynamic_cast<T> (v);
   gdb_assert (result != nullptr);
 #else