Replace all g_assert_not_reached() in app/core/ by g_return_if_reached()
or g_return_val_if_reached(). GIMP may handle a lot of creative work,
sometimes unsaved for hours. We should not just crash on purpose.
g_assert*() could theoretically be turned off on a glib build, but this
is nearly never done, and is not a solution either (actually it is
probably even worse because the broken code would just continue on a
forbidden path). It is much better to return with a warning on such
forbidden code paths, allowing someone to report a bug without
experiencing a crash and data loss.

For now, I only took care of g_assert_not_reached() inside app/core.
More g_assert*() code should be replaced.
Note: assert are acceptable in plug-ins though, but not in the main
executable, unless absolutely necessary (something happening so bad that
crash is better than continuing).