esplora: fix incorrect gap limit check in blocking client

The gap limit was checked such as if the last_index was not None but the
last_active_index was, we'd consider having reached it. But the
last_index is never None for this check. This effectively made it so the
gap limit was always 1: if the first address isn't used
last_active_index will be None and we'd return immediately.

Fix this by avoiding error-prone Option comparisons and correctly
checking we've reached the gap limit before breaking out of the loop.

crates/esplora/src/blocking_ext.rs

@@ -250,7 +250,8 @@ impl EsploraExt for esplora_client::BlockingClient {
                     }
                 }
 
-                if last_index > last_active_index.map(|i| i.saturating_add(stop_gap as u32)) {
+                let last_index = last_index.expect("Must be set since handles wasn't empty.");
+                if last_index > last_active_index.unwrap_or(0) + stop_gap as u32 {
                     break;
                 }
             }