staging: rtl8723bs: Prevent an underflow in rtw_check_beacon_data().

staging: rtl8723bs: Prevent an underflow in rtw_check_beacon_data().

Dan Carpenter reported an integer underflow issue in the rtl8188eu driver.
This is also needed for the length (signed integer) in rtl8723bs, as it is
later converted to an unsigned integer and used in a memcpy operation.

Original issue is at https://patchwork.kernel.org/patch/9796371/

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Murray McAllister <murray.mcallister@insomniasec.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>