library/GSM_SystemInformation: add dec_SystemInformationSafe()

Some types of System Information (mostly the Rest Octets) are not
fully implemented, so calling the generic dec_SystemInformation()
may result in a DTE.  Let's add dec_SystemInformationSafeBT() with
"prototype(backtrack)", so it would return a non-zero integer if
decoding fails.  Let's add a wrapper dec_SystemInformationSafe()
that would additionally check the RR Protocol Discriminator.

Change-Id: Id4d73e0f3347e1d4c4c77aec75b767311d662292
Related: OS#4662
2 files changed