usb: core: buffer: avoid NULL pointer dereferrence

NULL pointer dereferrence will happen when class driver
wants to allocate zero length buffer and pool_max[0]
can't be used, so simply returns NULL in the case.
Signed-off-by: NChunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/buffer.c

@@ -122,6 +122,9 @@ void *hcd_buffer_alloc(
 	struct usb_hcd		*hcd = bus_to_hcd(bus);
 	int			i;
 
+	if (size == 0)
+		return NULL;
+
 	/* some USB hosts just use PIO */
 	if (!IS_ENABLED(CONFIG_HAS_DMA) ||
 	    (!bus->controller->dma_mask &&