ENH: fail-safe access of ITstream tokens as front(), back()

applications/test/ITstream/Test-ITstream.C

src/OpenFOAM/db/IOstreams/Tstreams/ITstream.C

src/OpenFOAM/db/IOstreams/Tstreams/ITstream.H

src/OpenFOAM/db/dictionary/functionEntries/ifEntry/ifEntry.C

...b/functionObjects/functionObjectList/functionObjectList.C