tests: Need to initialize 'test' properly on error path

In the error path, the test buffer is free'd, but due to how the free
routine is written the 'test' buffer pointer does not return to the caller
as NULL and then the free'd buffer address is returned to the caller.

tests/qemumonitortestutils.c

@@ -536,6 +536,7 @@ no_memory:
 error:
     VIR_FREE(tmpdir_template);
     qemuMonitorTestFree(test);
+    test = NULL;
     goto cleanup;
 }