fuse: add missing fuse_request_free

The error handling code for the second call to fuse_request_alloc should
include freeing the result of the first one.

This bug was found by the Coccinelle project:

  http://www.emn.fr/x-info/coccinelle/

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>