patch 8.1.1053: warning for missing return statement

Problem:    Warning for missing return statement. (Dominique Pelle)
Solution:   Add return statement.
diff --git a/src/undo.c b/src/undo.c
index 4fd58a3..978549a 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -3541,6 +3541,7 @@
     FOR_ALL_BUFFERS(buf)
 	if (bufIsChanged(buf))
 	    return TRUE;
+    return FALSE;
 }
 
 /*