authz-list-file: Fix file read error handling

authz-list-file: Fix file read error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

qauthz_list_file_complete() is wrong that way: it passes @errp to
qauthz_list_file_complete() without checking for failure.  If it runs
into another failure, it trips error_setv()'s assertion.  Reproducer:

    $ qemu-system-x86_64 -nodefaults -S -display none -object authz-list-file,id=authz0,filename=
    qemu-system-x86_64: ../util/error.c:59: error_setv: Assertion `*errp == NULL' failed.
    Aborted (core dumped)

Fix it to check for failure.

Fixes: 55d869846de802a16af1a50584c51737bd664387
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>