patch 8.1.0470: pointer ownership around fname_expand() is unclear

Problem:    Pointer ownership around fname_expand() is unclear.
Solution:   Allow b_ffname and b_sfname to point to the same allocated memory,
            only free one.  Update comments.
diff --git a/src/version.c b/src/version.c
index 12270b7..3257bd2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    470,
+/**/
     469,
 /**/
     468,