stricter bounds checking in BitPumpPlain
stricter bounds checking in BitPumpPlain

BitPumpPlain allowed offset==size as a valid offset. Be stricter
about it. The FileMap margin should make this safe anyway but
better be careful anyway.

LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,klauspost/rawspeed,darktable-org/rawspeed,anarsoul/rawspeed,darktable-org/rawspeed,klauspost/rawspeed,anarsoul/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,anarsoul/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed