net: wireless: bcmdhd: Fix for arbitrary memory free.

Fix for arbitrary memory free in nexus6p's wifi driver
function wl_cfgvendor_dbg_get_mem_dump. Current fix
includes intialize mem_buf to NULL and check if the
len is valid or not. Also check if buf_len is valid
or not. If buf_len is not valid then mem_buf will be
set to NULL.

Signed-off-by: Sudhir Kohalli <sudhir.kohalli@broadcom.com>

Change-Id: Ia98ce18f0437d38d6f6d77033af7477ae12574e3
Bug: 34624457