patch 8.2.1377: triggering the ATTENTION prompt causes typeahead mess up

Problem:    Triggering the ATTENTION prompt causes typeahead to be messed up.
Solution:   Increment tb_change_cnt. (closes #6541)
diff --git a/src/getchar.c b/src/getchar.c
index e040748..a8778e0 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -447,6 +447,8 @@
     typebuf.tb_silent = 0;
     cmd_silent = FALSE;
     typebuf.tb_no_abbr_cnt = 0;
+    if (++typebuf.tb_change_cnt == 0)
+	typebuf.tb_change_cnt = 1;
 }
 
 /*