patch 8.2.4111: potential proglem when map is deleted while executing

Problem:    Potential proglem when map is deleted while executing.
Solution:   Reset last used map pointer when deleting a mapping.
diff --git a/src/map.c b/src/map.c
index 181aa65..6ad938d 100644
--- a/src/map.c
+++ b/src/map.c
@@ -85,6 +85,9 @@
     vim_free(mp->m_orig_str);
     *mpp = mp->m_next;
     vim_free(mp);
+#ifdef FEAT_EVAL
+    reset_last_used_map();
+#endif
 }
 
 /*