patch 8.2.2670: Vim9: error for append(0, text)

Problem:    Vim9: error for append(0, text).
Solution:   Check for negative number. (closes #8022)
diff --git a/src/typval.c b/src/typval.c
index 8030c69..ca561dd 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -1621,7 +1621,7 @@
 
     if (argvars[0].v_type != VAR_STRING || !in_vim9script())
 	lnum = (linenr_T)tv_get_number_chk(&argvars[0], NULL);
-    if (lnum <= 0)  // no valid number, try using arg like line()
+    if (lnum < 0)  // no valid number, try using arg like line()
     {
 	int	fnum;
 	pos_T	*fp = var2fpos(&argvars[0], TRUE, &fnum, FALSE);