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

commit 920c92448839bd4f8eb87a92b08cad56d449caff upstream.

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>