patch 8.2.0082: when reusing a buffer listeners are not cleared

Problem:    When reusing a buffer listeners are not cleared. (Axel Forsman)
Solution:   Clear listeners when reusing a buffer. (closes #5431)
diff --git a/src/buffer.c b/src/buffer.c
index eb6b287..0a68d48 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -979,6 +979,7 @@
 	hash_init(&buf->b_vars->dv_hashtab);
 	init_changedtick(buf);
 	CHANGEDTICK(buf) = tick;
+	remove_listeners(buf);
     }
 #endif
     uc_clear(&buf->b_ucmds);		// clear local user commands