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.

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    470,
 /**/
     469,
 /**/