Zero-fill new GtkTextIter

Zero-fill new GtkTextIter

iter_init_common() is used on uninitialized GtkTextIter, and since neither it
nor its callers initiliaze its padding fields, they contain garbage.

This is a problem for Go - which checks that structs passed to C functions do
not contain pointers to Go-allocated memory - when the garbage happens to be
such a pointer.  Although Go zero-fills all GtkTextIter that it allocates, this
does not help when GTK functions such as insert_pixbuf_or_widget_segment called
for gtk_text_buffer_create_child_anchor copy garbage from their stack-allocated
GtkTextIter into a clean iter.  To work around this a GtkTextIter has to be
discraded after use in text buffer anchor inserting functions:
https://github.com/gotk3/gotk3/pull/307