textbuffer: nicer get_iter functions, without return values

textbuffer: nicer get_iter functions, without return values

Avoid crashes when passing an invalid location to a
gtk_text_buffer_get_iter_at_*() function.

A first attempt added boolean return values to know if @iter has been set to
the exact location, but it breaks Python and JS bindings because the out
parameter is already a return value in those languages.

Unit tests are added.

https://bugzilla.gnome.org/show_bug.cgi?id=735341