cpu: verify that block->host is set

If it isn't, access at an offset will cause memory corruption.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>

include/exec/cpu-all.h

@@ -316,6 +316,7 @@ typedef struct RAMBlock {
 static inline void *ramblock_ptr(RAMBlock *block, ram_addr_t offset)
 {
     assert(offset < block->length);
+    assert(block->host);
     return (char *)block->host + offset;
 }