block/nfs: fix NULL pointer dereference in URI parsing

parse_uint_full wants to put the parsed value into the
variable passed via its second argument which is NULL.

Fixes: 94d6a7a7
Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1485942829-10756-2-git-send-email-pl@kamp.de
Signed-off-by: Max Reitz <mreitz@redhat.com>