qemuhotplugtest: Free monitor iff successfully initialized

If initializing test monitor in testQemuHotplugCpuPrepare()
fails, the control jumps to error label where
testQemuHotplugCpuDataFree() is called. But since the data->mon
is NULL due to aforementioned failure,
qemuMonitorTestGetMonitor() dereferences a NULL pointer leading
to a SIGSEGV.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

tests/qemuhotplugtest.c

@@ -399,9 +399,11 @@ testQemuHotplugCpuDataFree(struct testQemuHotplugCpuData *data)
         virObjectUnref(data->vm);
     }
 
-    mon = qemuMonitorTestGetMonitor(data->mon);
-    virObjectLock(mon);
-    qemuMonitorTestFree(data->mon);
+    if (data->mon) {
+        mon = qemuMonitorTestGetMonitor(data->mon);
+        virObjectLock(mon);
+        qemuMonitorTestFree(data->mon);
+    }
     VIR_FREE(data);
 }