hw: Don't call visit_end_struct() after visit_start_struct() fails

When visit_start_struct() fails, visit_end_struct() must not be
called.  rtc_get_date() and balloon_stats_all() call it anyway.  As
far as I can tell, they're only used with the string output visitor,
which doesn't care.  Fix them anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

hw/timer/mc146818rtc.c

@@ -793,19 +793,26 @@ static const MemoryRegionOps cmos_ops = {
 static void rtc_get_date(Object *obj, Visitor *v, void *opaque,
                          const char *name, Error **errp)
 {
+    Error *err = NULL;
     RTCState *s = MC146818_RTC(obj);
     struct tm current_tm;
 
     rtc_update_time(s);
     rtc_get_time(s, &current_tm);
-    visit_start_struct(v, NULL, "struct tm", name, 0, errp);
-    visit_type_int32(v, &current_tm.tm_year, "tm_year", errp);
-    visit_type_int32(v, &current_tm.tm_mon, "tm_mon", errp);
-    visit_type_int32(v, &current_tm.tm_mday, "tm_mday", errp);
-    visit_type_int32(v, &current_tm.tm_hour, "tm_hour", errp);
-    visit_type_int32(v, &current_tm.tm_min, "tm_min", errp);
-    visit_type_int32(v, &current_tm.tm_sec, "tm_sec", errp);
-    visit_end_struct(v, errp);
+    visit_start_struct(v, NULL, "struct tm", name, 0, &err);
+    if (err) {
+        goto out;
+    }
+    visit_type_int32(v, &current_tm.tm_year, "tm_year", &err);
+    visit_type_int32(v, &current_tm.tm_mon, "tm_mon", &err);
+    visit_type_int32(v, &current_tm.tm_mday, "tm_mday", &err);
+    visit_type_int32(v, &current_tm.tm_hour, "tm_hour", &err);
+    visit_type_int32(v, &current_tm.tm_min, "tm_min", &err);
+    visit_type_int32(v, &current_tm.tm_sec, "tm_sec", &err);
+    visit_end_struct(v, &err);
+
+out:
+    error_propagate(errp, err);
 }
 
 static void rtc_realizefn(DeviceState *dev, Error **errp)

hw/virtio/virtio-balloon.c

@@ -108,6 +108,7 @@ static void balloon_stats_poll_cb(void *opaque)
 static void balloon_stats_get_all(Object *obj, struct Visitor *v,
                                   void *opaque, const char *name, Error **errp)
 {
+    Error *err = NULL;
     VirtIOBalloon *s = opaque;
     int i;
 
@@ -116,17 +117,29 @@ static void balloon_stats_get_all(Object *obj, struct Visitor *v,
         return;
     }
 
-    visit_start_struct(v, NULL, "guest-stats", name, 0, errp);
-    visit_type_int(v, &s->stats_last_update, "last-update", errp);
+    visit_start_struct(v, NULL, "guest-stats", name, 0, &err);
+    if (err) {
+        goto out;
+    }
+
+    visit_type_int(v, &s->stats_last_update, "last-update", &err);
 
-    visit_start_struct(v, NULL, NULL, "stats", 0, errp);
+    visit_start_struct(v, NULL, NULL, "stats", 0, &err);
+    if (err) {
+        goto out_end;
+    }
+        
     for (i = 0; i < VIRTIO_BALLOON_S_NR; i++) {
         visit_type_int64(v, (int64_t *) &s->stats[i], balloon_stat_names[i],
-                         errp);
+                         &err);
     }
-    visit_end_struct(v, errp);
+    visit_end_struct(v, &err);
+
+out_end:
+    visit_end_struct(v, &err);
 
-    visit_end_struct(v, errp);
+out:
+    error_propagate(errp, err);
 }
 
 static void balloon_stats_get_poll_interval(Object *obj, struct Visitor *v,