[Bug c++/12277] Warn on dynamic casts with known NULL results.

[gcc at dixie dot net.nz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

--- Comment #8 from Andrew Dixie <gcc at dixie dot net.nz> ---
Created attachment 34130
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34130&action=edit
dynamic_cast warning