wifi: ath12k: fix possible out-of-bound read in ath12k_htt_pull_ppdu_stats()

In the Linux kernel, the following vulnerability has been resolved: wifi: ath12k: fix possible out-of-bound read in ath12k_htt_pull_ppdu_stats() len is extracted from HTT message and could be an unexpected value in case errors happen, so add validation before using to avoid possible out-of-bound read in the following message iteration and parsing. The same issue also applies to ppdu_info->ppdu_stats.common.num_users, so validate it before using too. These are found during code review. Compile test only.