patch 9.1.0648: [security] double-free in dialog_changed()

Problem:  [security] double-free in dialog_changed()
          (SuyueGuo)
Solution: Only clear pointer b_sfname pointer, if it is different
          than the b_ffname pointer.  Don't try to free b_fname,
          set it to NULL instead.

fixes: #15403

Github Advisory:
https://github.com/vim/vim/security/advisories/GHSA-46pw-v7qw-xc2f

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/ex_cmds2.c

@@ -197,9 +197,11 @@ dialog_changed(
 	// restore to empty when write failed
 	if (empty_bufname)
 	{
-	    VIM_CLEAR(buf->b_fname);
-	    VIM_CLEAR(buf->b_ffname);
+	    // prevent double free
+	    if (buf->b_sfname != buf->b_ffname)
 		VIM_CLEAR(buf->b_sfname);
+	    buf->b_fname = NULL;
+	    VIM_CLEAR(buf->b_ffname);
 	    unchanged(buf, TRUE, FALSE);
 	}
     }

src/testdir/test_crash.vim

@@ -196,6 +196,12 @@ func Test_crash1_3()
   call term_sendkeys(buf, args)
   call TermWait(buf, 50)
 
+  let file = 'crash/dialog_changed_uaf'
+  let cmn_args = "%s -u NONE -i NONE -n -e -s -S %s -c ':qa!'\<cr>"
+  let args = printf(cmn_args, vim, file)
+  call term_sendkeys(buf, args)
+  call TermWait(buf, 150)
+
   " clean up
   exe buf .. "bw!"
   bw!

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    648,
 /**/
     647,
 /**/