patch 8.1.0897: can modify a:000 when using a reference

Problem:    Can modify a:000 when using a reference.
Solution:   Make check for locked variable stricter. (Ozaki Kiichi,
            closes #3930)
diff --git a/src/userfunc.c b/src/userfunc.c
index d1ea33a..a293dd6 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2394,11 +2394,11 @@
 	if (fudi.fd_di == NULL)
 	{
 	    /* Can't add a function to a locked dictionary */
-	    if (tv_check_lock(fudi.fd_dict->dv_lock, eap->arg, FALSE))
+	    if (var_check_lock(fudi.fd_dict->dv_lock, eap->arg, FALSE))
 		goto erret;
 	}
 	    /* Can't change an existing function if it is locked */
-	else if (tv_check_lock(fudi.fd_di->di_tv.v_lock, eap->arg, FALSE))
+	else if (var_check_lock(fudi.fd_di->di_tv.v_lock, eap->arg, FALSE))
 	    goto erret;
 
 	/* Give the function a sequential number.  Can only be used with a