patch 9.0.0873: using freed memory when executing mapclear at more prompt

Problem:    Using freed memory when executing mapclear at the more prompt.
Solution:   Do not clear mappings while listing them. (closes #11438)
diff --git a/src/errors.h b/src/errors.h
index ea33b04..88db285 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3333,3 +3333,5 @@
 EXTERN char e_cannot_resize_window_in_another_tab_page[]
 	INIT(= N_("E1308: Cannot resize a window in another tab page"));
 #endif
+EXTERN char e_cannot_change_mappings_while_listing[]
+	INIT(= N_("E1309: Cannot change mappings while listing"));