backends/dbus-vmstate: Fix short read error handling

When dbus_vmstate_post_load() fails, it complains to stderr.  Except
on short read, where it checks with g_return_val_if_fail().  This
fails silently if G_DISABLE_CHECKS is undefined (it should be), or
else pads the short read with uninitialized bytes.

Replace g_return_val_if_fail() by a proper error check.

Fixes: 5010cec2bc87dafab39b3913c8ca91f88df9c540
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210126124240.2081959-2-armbru@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

backends/dbus-vmstate.c

@@ -229,7 +229,10 @@ static int dbus_vmstate_post_load(void *opaque, int version_id)
                                      &bytes_read, NULL, &err)) {
             goto error;
         }
-        g_return_val_if_fail(bytes_read == len, -1);
+        if (bytes_read != len) {
+            error_report("%s: Short read", __func__);
+            return -1;
+        }
         id[len] = 0;
 
         trace_dbus_vmstate_loading(id);