gNMI Server: Fix resource leak in SyncResponse send.
gNMI Server: Fix resource leak in SyncResponse send.

This seems to have been broken in cr/162791703. Formerly an error
was returned instead of the break. The calling function waits for
an error or nil to be written to c.errC.

PiperOrigin-RevId: 257714633

openconfig/gnmi,openconfig/gnmi,openconfig/gnmi