Calling NULL function pointer initiates panic loop

["Samuel J. Greear" <sjg@xxxxxxxxxxxx>]

Further description in the commit message,

http://gitweb.dragonflybsd.org/~sjg/dragonfly.git/log/refs/heads/null_fp

Please review, I am not sure this is necessarily the best way to
address this, for instance, you don't actually see where the panic
occurred.

Sam