wlan: Add sanity check for data in ptt_sock_rx_nlink_msg

Currently in the driver, while typecasting the skb data to local
structure in function ptt_sock_proc_reg_req, there can occur a potential
OOB read. The length of the data to be typecasted is not verified
properly.

Add a sanity check to verify that the data being typecasted is of proper
size.

Change-Id: Ib88477a2817649e092e25e21f948c33160dfa09b
CRs-Fixed: 2559499