exec: assert that RAMBlock size is non-zero

exec: assert that RAMBlock size is non-zero

find_ram_offset() does not handle size=0 gracefully.  It hands out the
same RAMBlock offset multiple times, leading to obscure failures later
on.

Add an assert to warn early if something is incorrectly allocating a
zero size RAMBlock.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>