Avoid assertion violations when using marker positions

Avoid assertion violations when using marker positions

* src/intervals.c (set_point_from_marker): If MARKER comes from
another buffer, recalculate its byte position before using it to
set point.
* src/marker.c (set_marker_internal): If POSITION is a marker from
another buffer, recalculate its byte position before using it.
(Bug#24368)