Allow --reindex-last-blocks to be larger than height

src/chain.rs

@@ -58,13 +58,12 @@ impl Chain {
         if n == 0 {
             return;
         }
-        if let Some(new_height) = self.height().checked_sub(n) {
+        let new_height = self.height().saturating_sub(n);
         self.update(vec![NewHeader::from((
             self.headers[new_height].1,
             new_height,
         ))])
     }
-    }
 
     /// Load the chain from a collecion of headers, up to the given tip
     pub(crate) fn load(&mut self, headers: Vec<BlockHeader>, tip: BlockHash) {