wifi: ath12k: Fix index decrement when array_len is zero

wifi: ath12k: Fix index decrement when array_len is zero

Currently, print_array_to_buf_index() decrements index unconditionally.
This may lead to invalid buffer access when array_len is zero.

Fix this by decrementing index only when array_len is non-zero.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.5-01651-QCAHKSWPL_SILICONZ-1
Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.1.c5-00302-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.115823.3

Fixes: adf6df963c03 ("wifi: ath12k: Add support to parse requested stats_type")
Signed-off-by: Aaradhana Sahu <aaradhana.sahu@oss.qualcomm.com>
Reviewed-by: Baochen Qiang <baochen.qiang@oss.qualcomm.com>
Link: https://patch.msgid.link/20260123071253.2202644-2-aaradhana.sahu@oss.qualcomm.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>