In this case conn->server_binding will be NULL and consequently
bind->Handle will be NULL, causing I_RpcFreeBuffer to fail. Therefore
just use I_RpcFree to free the buffer as that is just what
I_RpcFreeBuffer does under the hood anyway.

Add tests for this.

rpcrt4: Free the memory for a registered interface that is in use and when WaitForCallsToComplete is 0.

Based on a patch by Rob Shearman.