Use error_is_set() only when necessary (again)

error_is_set(&var) is the same as var != NULL, but it takes
whole-program analysis to figure that out.  Unnecessarily hard for
optimizers, static checkers, and human readers.  Commit 84d18f0 dumbed
it down to obvious, but a few more have crept in since, and
documentation was overlooked.  Dumb these down, too.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/block/quorum.c b/block/quorum.c
index 7f580a8..ecec3a5 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -753,7 +753,7 @@
 
     opts = qemu_opts_create(&quorum_runtime_opts, NULL, 0, &error_abort);
     qemu_opts_absorb_qdict(opts, options, &local_err);
-    if (error_is_set(&local_err)) {
+    if (local_err) {
         ret = -EINVAL;
         goto exit;
     }
@@ -828,7 +828,7 @@
     g_free(opened);
 exit:
     /* propagate error */
-    if (error_is_set(&local_err)) {
+    if (local_err) {
         error_propagate(errp, local_err);
     }
     QDECREF(list);