segfault when destroying GtkTextChildAnchors

[muppet <scott asofyet org>]

as seen in bugzilla: http://bugzilla.gnome.org/show_bug.cgi?id=132260

this kind of code results in a segfault:

	anchor = gtk_text_child_anchor_new ();
	g_object_unref (anchor);

basically, you decide not to use a new anchor and destroy it before 
putting it into a textbuffer.  anchor->segment doesn't get set, and 
gtk_text_child_anchor_finalize() barfs all over itself when it tries to 
dereference NULL.

i created a patch that just checks for NULL before doing anything with 
segment, and it seems to work fine.

however, it seems so obvious, and has never been a problem before, that 
i wonder if i'm doing something that i really shouldn't have been 
doing.  is it invalid to create a child anchor without a text buffer?

--
Brian: If i recall correctly, this is the physics department.
Chris: That explains all that gravity.
	-- Family Guy, "The Story on Page One"