revealer: Don't report >0 preferred width...

revealer: Don't report >0 preferred width...

... if the child is concealed and the transition type is set to NONE.
In this case, both preferred width and preferred height should be 0, but
we still can't use that to allocate a size for the child, so care about
the minimum size of the child in gtk_revealer_get_child_allocation.

https://bugzilla.gnome.org/show_bug.cgi?id=761760