inspector: fix a crash in the CSS editor

inspector: fix a crash in the CSS editor

gtk_css_section_get_end_position() can return a position one byte beyond
the end of the last line.

gtk_text_buffer_get_iter_at_line_index() accepts only valid
line_number/byte_index pairs. Another solution is to make the
GtkTextBuffer function less strict, by returning a boolean if the exact
position was found.

The CSS parser should also be fixed to always return valid positions.
But it's better to have a safety net in the CSS editor, just in case (a
warning could be print).

https://bugzilla.gnome.org/show_bug.cgi?id=735337