Drivers: hv : vmbus: Adding NULL pointer check

This patch fixes the following Coccinelle warning:
drivers/hv/ring_buffer.c:223: alloc with no test

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Link: https://lore.kernel.org/r/20211028104138.14576-1-lv.ruyi@zte.com.cn


Signed-off-by: Wei Liu <wei.liu@kernel.org>