vkGetDeviceQueue2: add a NULL check

The current driver::GetDeviceQueue2() will indeed crash if the
driver returns a NULL queue, which it is supposed to do if the
pQueueInfo->flags doesn't match the flags requested for the queue
family. Thus need to add a NULL check for pQueue here in case.

Test: deqp vulkancts test
Bug: b/72867433
Change-Id: I306c1a1db52a9986801357eaeb357536ecb0428e