qom: Don't keep error value between object_property_parse() calls

When handling errp==NULL at object_apply_global_props(), we are
leaving the old error value in `err` after printing a warning.
This makes QEMU crash if two global properties generate warnings:

  $ echo device_add rtl8139 | qemu-system-x86_64 -monitor stdio -global rtl8139.xxx=yyy -global rtl8139.xxx=zzz
  warning: can't apply global rtl8139.xxx=yyy: Property '.xxx' not found
  qemu-system-x86_64: util/error.c:57: error_setv: Assertion `*errp == NULL' failed.
  Aborted (core dumped)

Fix that by making `err` go out of scope immediately after the
warn_report_err() call.

Fixes: 50545b2c "qdev-props: call object_apply_global_props()"
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190110020259.8492-2-ehabkost@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

qom/object.c

@@ -372,7 +372,6 @@ static void object_post_init_with_type(Object *obj, TypeImpl *ti)
 
 void object_apply_global_props(Object *obj, const GPtrArray *props, Error **errp)
 {
-    Error *err = NULL;
     int i;
 
     if (!props) {
@@ -381,6 +380,7 @@ void object_apply_global_props(Object *obj, const GPtrArray *props, Error **errp
 
     for (i = 0; i < props->len; i++) {
         GlobalProperty *p = g_ptr_array_index(props, i);
+        Error *err = NULL;
 
         if (object_dynamic_cast(obj, p->driver) == NULL) {
             continue;