tune2fs: exit directly when fs freed in ext2fs_run_ext3_journal

tune2fs: exit directly when fs freed in ext2fs_run_ext3_journal

In ext2fs_run_ext3_journal(), fs will be freed and reallocated.
However, the reallocation by ext2fs_open() may fail in some cases ---
for example, when the device becomes offline.  To avoid a segfault,
exit if fs is NULL.

[ Simplified the patch by by simply exiting if fs is NULL -TYT ]

Signed-off-by: Li Jinlin <lijinlin3@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>