2002-04-28  Sven Neumann  <sven@gimp.org>

	* app/core/gimpimage-convert.c (gimp_image_convert): invalidate layer
	previews so we don't crash when an image preview is generated after
	converting the image from RGB to GRAY.

	* app/core/gimpimage.c (gimp_image_get_new_preview): added some
	assertions to make it easier to find similar problems in the future.