don't return a state in state_at if the db prunes and the block is before guaranteed history

ethcore/src/client/client.rs

@@ -371,6 +371,14 @@ impl<V> Client<V> where V: Verifier {
 			return Some(self.state())
 		}
 
+		let block_number = self.block_number(id.clone());
+
+		// check that the block is not too old -- blocks within `HISTORY` blocks of the best will
+		// always be available.
+		if self.state_db.does_pruning() && self.best_block_number() >= block_number + HISTORY {
+			return None;
+		}
+
 		self.block_header(id).map(|header| {
 			let db = self.state_db.lock().unwrap().boxed_clone();
 			State::from_existing(db, HeaderView::new(&header).state_root(), self.engine.account_start_nonce())