radv: vkAllocateCommandBuffers should NULL all output handles

This is part of the spec and fixes CTS tests:
dEQP-VK.api.object_management.alloc_callback_fail_multiple.command_buffer_*

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/amd/vulkan/radv_cmd_buffer.c

@@ -1351,6 +1351,9 @@ VkResult radv_AllocateCommandBuffers(
 	VkResult result = VK_SUCCESS;
 	uint32_t i;
 
+	memset(pCommandBuffers, 0,
+			sizeof(*pCommandBuffers)*pAllocateInfo->commandBufferCount);
+
 	for (i = 0; i < pAllocateInfo->commandBufferCount; i++) {
 		result = radv_create_cmd_buffer(device, pool, pAllocateInfo->level,
 						&pCommandBuffers[i]);