rpcrt4/tests: Only free string when function succeeded.

dlls/rpcrt4/tests/rpc.c

@@ -283,8 +283,7 @@ todo_wine {
     ok(level == RPC_C_AUTHN_LEVEL_PKT_PRIVACY, "expected RPC_C_AUTHN_LEVEL_PKT_PRIVACY, got %d\n", level);
     ok(authnsvc == RPC_C_AUTHN_WINNT, "expected RPC_C_AUTHN_WINNT, got %d\n", authnsvc);
     todo_wine ok(authzsvc == RPC_C_AUTHZ_NAME, "expected RPC_C_AUTHZ_NAME, got %d\n", authzsvc);
-
-    RpcStringFree(&principal);
+    if (status == RPC_S_OK) RpcStringFree(&principal);
 
     status = RpcMgmtStopServerListening(NULL);
     ok(status == RPC_S_OK, "RpcMgmtStopServerListening failed (%u)\n",