qapi: Clean up fragile use of error_is_set()

Using error_is_set(ERRP) to find out whether a function failed is
either wrong, fragile, or unnecessarily opaque.  It's wrong when ERRP
may be null, because errors go undetected when it is.  It's fragile
when proving ERRP non-null involves a non-local argument.  Else, it's
unnecessarily opaque (see commit 84d18f06).

The error_is_set(errp) in do_qmp_dispatch() is merely fragile, because
the caller never passes a null errp argument.

Make the code more robust and more obviously correct: receive the
error in a local variable, then propagate it through the parameter.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

qapi/qmp-dispatch.c

@@ -62,6 +62,7 @@ static QDict *qmp_dispatch_check_obj(const QObject *request, Error **errp)
 
 static QObject *do_qmp_dispatch(QObject *request, Error **errp)
 {
+    Error *local_err = NULL;
     const char *command;
     QDict *args, *dict;
     QmpCommand *cmd;
@@ -93,13 +94,13 @@ static QObject *do_qmp_dispatch(QObject *request, Error **errp)
 
     switch (cmd->type) {
     case QCT_NORMAL:
-        cmd->fn(args, &ret, errp);
-        if (!error_is_set(errp)) {
-            if (cmd->options & QCO_NO_SUCCESS_RESP) {
-                g_assert(!ret);
-            } else if (!ret) {
-                ret = QOBJECT(qdict_new());
-            }
+        cmd->fn(args, &ret, &local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+        } else if (cmd->options & QCO_NO_SUCCESS_RESP) {
+            g_assert(!ret);
+        } else if (!ret) {
+            ret = QOBJECT(qdict_new());
         }
         break;
     }