power: fix sanity checks for guest channel read

power: fix sanity checks for guest channel read

In function power_guest_channel_read_msg, 'lcore_id' is used before
validity check, which may cause buffer 'global_fds' accessed by index
'lcore_id' overflow.

This patch moves the validity check of 'lcore_id' before the 'lcore_id'
being used for the first time.

Fixes: 9dc843eb273b ("power: extend guest channel API for reading")
Cc: stable@dpdk.org
Signed-off-by: Hongbo Zheng <zhenghongbo3@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Reviewed-by: Reshma Pattan <reshma.pattan@intel.com>
Acked-by: David Hunt <david.hunt@intel.com>