SAP: Fix logic to skip padding bytes for requests received

As per SAP spec, padding bytes can be 0-3 bytes, but the current code
has incorrect logic, that would lead to calculation of padding bytes
as 4 in case the APDU length was multiple of 4, which would lead to
incorrect parsing logic in SAP Server and can lead to issues of no
response for APDU request from SAP Server as it keeps waiting for
reading more bytes from rfcomm which are not present.

Also check added in code not to send msg to RIL if socket is null to
prevent crash in BT Sap module.

Bug: 23024598
Signed-off-by: Casper Bonde <c.bonde@samsung.com>
Change-Id: I24e4a6b850709c9c32b7e0992626a0219dacef03