patch 8.2.1477: Vim9: error when using bufnr('%')

Problem:    Vim9: error when using bufnr('%').
Solution:   Don't give an error for using a string argument. (closes #6723)
diff --git a/src/evalbuffer.c b/src/evalbuffer.c
index df5270a..2d738f1 100644
--- a/src/evalbuffer.c
+++ b/src/evalbuffer.c
@@ -390,7 +390,8 @@
 	buf = curbuf;
     else
     {
-	(void)tv_get_number(&argvars[0]);    // issue errmsg if type error
+	if (argvars[0].v_type != VAR_STRING)
+	    (void)tv_get_number(&argvars[0]);    // issue errmsg if type error
 	++emsg_off;
 	buf = tv_get_buf(&argvars[0], FALSE);
 	--emsg_off;