Fix crash free'ing securityDriverNames in QEMU driver

The previous fix to avoid leaking securityDriverNames forgot to
handle the case of securityDriverNames being NULL, leading to
a crash
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/qemu/qemu_driver.c

@@ -1134,7 +1134,8 @@ qemuShutdown(void) {
     VIR_FREE(qemu_driver->saveImageFormat);
     VIR_FREE(qemu_driver->dumpImageFormat);
 
-    for (i = 0 ; qemu_driver->securityDriverNames[i] != NULL ; i++)
+    for (i = 0 ; (qemu_driver->securityDriverNames != NULL &&
+                  qemu_driver->securityDriverNames[i] != NULL) ; i++)
         VIR_FREE(qemu_driver->securityDriverNames[i]);
     VIR_FREE(qemu_driver->securityDriverNames);
     virSecurityManagerFree(qemu_driver->securityManager);