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

exec.c

@@ -925,6 +925,8 @@ static ram_addr_t find_ram_offset(ram_addr_t size)
     RAMBlock *block, *next_block;
     ram_addr_t offset = RAM_ADDR_MAX, mingap = RAM_ADDR_MAX;
 
+    assert(size != 0); /* it would hand out same offset multiple times */
+
     if (QTAILQ_EMPTY(&ram_list.blocks))
         return 0;