exofs: check for allocation failure in uri_store()

There is no memory allocation failure check in uri_store().
That can lead to NULL pointer dereference.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>