wlan: Fix possible OOB access in lim_process_disassoc_frame

Reason code is extracted from frame data without validating
the frame len which could result in out of bound access.

Fix is to validate frame len before extracting reason
code from frame data.

Change-Id: I00795a806abcae903dd0daa019aeab990aedc3a7
CRs-Fixed: 2504023