fuse: prevent fuse_put_request on invalid pointer

fuse_direct_io() has a loop where requests are allocated in each
iteration. if allocation fails, the loop is broken out and follows
into an unconditional fuse_put_request() on that invalid pointer.

Signed-off-by: Anand V. Avati <avati@gluster.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: stable@kernel.org