segv on mistaken bless

[Kevin Ryde <user42 zip com au>]

I made a mistake in a GObject subclass creation by just blessing into
the class instead of Glib::Object->new and got a segv from signal_emit,
per below.

It'd be nice if the code noticed that, but I couldn't tell if it'd be
the signal_emit xsub or gperl_get_object_check which might check for a
missing mg_find.  Do some uses of gperl_get_object_check expect to get
back NULL for undef?

package Foo;
use strict;
use warnings;
use Glib;

use Glib::Object::Subclass
  Glib::Object::,
  signals => { message => { param_types => ['Glib::String'],
                            return_type => undef },
             };

sub start {
  my ($class, %self) = @_;
  my $self = bless \%self, $class;
}

package main;
use strict;
use warnings;

my $foo = Foo->start ();
$foo->signal_emit ('message', 'hello world');
exit 0;