patch 8.2.3919: Vim9: wrong argument for append() results in two errors

Problem:    Vim9: wrong argument for append() results in two errors.
Solution:   Check did_emsg.  Also for setline().  Adjust the help for
            appendbufline().
diff --git a/src/typval.c b/src/typval.c
index 2f773b6..34ff47c 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -2249,9 +2249,10 @@
     if (lnum <= 0 && argvars[0].v_type != VAR_NUMBER)
     {
 	int	fnum;
-	pos_T	*fp = var2fpos(&argvars[0], TRUE, &fnum, FALSE);
+	pos_T	*fp;
 
 	// no valid number, try using arg like line()
+	fp = var2fpos(&argvars[0], TRUE, &fnum, FALSE);
 	if (fp != NULL)
 	    lnum = fp->lnum;
     }
@@ -2269,6 +2270,7 @@
     if (argvars[0].v_type == VAR_STRING
 	    && argvars[0].vval.v_string != NULL
 	    && argvars[0].vval.v_string[0] == '$'
+	    && argvars[0].vval.v_string[1] == NUL
 	    && buf != NULL)
 	return buf->b_ml.ml_line_count;
     return (linenr_T)tv_get_number_chk(&argvars[0], NULL);