Add context id getter to TKM nonce generator

src/charon-tkm/tests/nonceg_tests.c

@@ -26,6 +26,7 @@ START_TEST(test_nonceg_creation)
 
 	ng = tkm_nonceg_create();
 	fail_if(ng == NULL, "Error creating tkm nonce generator");
+	fail_if(ng->get_id(ng) == 0, "Invalid context id (0)");
 
 	ng->nonce_gen.destroy(&ng->nonce_gen);
 }