MonitorDBStore: make sure the provided bl is empty

If it's not, data will be appended to the end of bufferlist and difficult
to spot bugs might arise, so fail hard in that case.

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>

src/mon/MonitorDBStore.h

@@ -507,6 +507,7 @@ class MonitorDBStore
   }
 
   int get(const string& prefix, const string& key, bufferlist& bl) {
+    assert(bl.empty());
     return db->get(prefix, key, &bl);
   }