fix rescan crash: shutdown old lightclient before overwriting

lib/src/lib.rs

@@ -140,6 +140,17 @@ pub extern "C" fn litelib_initialize_new_from_phrase(dangerous: bool, server: *c
 
     //println!("Initializing with server: {}, seed: {}", server_str, seed_str);
 
+    // Shut down the existing client if one is running, to stop background threads
+    if overwrite {
+        let old_lc = match LIGHTCLIENT.lock() {
+            Ok(l) => l.borrow().clone(),
+            Err(poisoned) => poisoned.into_inner().borrow().clone(),
+        };
+        if let Some(lc) = old_lc {
+            lc.shutdown();
+        }
+    }
+
     let server = LightClientConfig::get_server_or_default(Some(server_str));
     let (config, _latest_block_height) = match LightClientConfig::create(server, dangerous) {
         Ok((c, h)) => {