patch 8.2.1571: Vim9: count() third argument cannot be "true"

Problem:    Vim9: count() third argument cannot be "true".
Solution:   use tv_get_bool_chk(). (closes #6818)
diff --git a/src/list.c b/src/list.c
index e7d288a..e0c8c2e 100644
--- a/src/list.c
+++ b/src/list.c
@@ -2167,7 +2167,7 @@
     int		error = FALSE;
 
     if (argvars[2].v_type != VAR_UNKNOWN)
-	ic = (int)tv_get_number_chk(&argvars[2], &error);
+	ic = (int)tv_get_bool_chk(&argvars[2], &error);
 
     if (argvars[0].v_type == VAR_STRING)
     {