textiter: fix bug in gtk_text_iter_backward_line()

gtk_text_iter_backward_line() checks the value of
real->line_char_offset without previously calling
ensure_char_offsets (real) to make sure the former
is up-to-date.

As a consequence of this, when gtk_text_iter_backward_line()
is called after a gtk_text_buffer_insert_range() in the
first line of buffer, the iter is not moved to the start of
the line, and the return value is wrong.

Fixed by adding the ensure_char_offsets() call.

gtk/gtktextiter.c

@@ -2547,6 +2547,8 @@ gtk_text_iter_backward_line (GtkTextIter *iter)
   if (real == NULL)
     return FALSE;
 
+  ensure_char_offsets (real);
+
   check_invariants (iter);
 
   new_line = _gtk_text_line_previous (real->line);