Ticket #2535: segfault in FTP VFS

...because of uninitialized pointer usage. The bug was introduced
in 68628184b1335276998ee0fecb3b995b64c74723 commit.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

lib/vfs/direntry.c

@@ -1198,6 +1198,7 @@ vfs_s_open (struct vfs_class *me, const char *file, int flags, mode_t mode)
     fh->handle = -1;
     fh->changed = was_changed;
     fh->linear = 0;
+    fh->data = NULL;
 
     if (IS_LINEAR (flags))
     {