wlcore: check minimum buffer size in some cmd_send functions

Check for the minimum required buffer length in wlcore_cmd_send() and
wlcore_cmd_configure_failsafe.  This ensures that we will never try to
use a buffer that is smaller than the required header.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>