minor robustness improvement in IFF image reading code
minor robustness improvement in IFF image reading code

corresponds to Perforce change 6081.

LogicalKnight/povray,FLOWERCLOUD/povray,FLOWERCLOUD/povray,wfpokorny/povray,wfpokorny/povray,c-lipka/povray,eblanca/povray,LeForgeron/povray,POV-Ray/povray,c-lipka/povray,c-lipka/povray,UberPOV/UberPOV,LeForgeron/povray,BentSm/povray,LogicalKnight/povray,BentSm/povray,LeForgeron/povray,BentSm/povray,POV-Ray/povray,LeForgeron/povray,davidgiven/povray,LogicalKnight/povray,davidgiven/povray,FLOWERCLOUD/povray,LeForgeron/povray,POV-Ray/povray,LeForgeron/povray,POV-Ray/povray,UberPOV/UberPOV,davidgiven/povray,LogicalKnight/povray,FLOWERCLOUD/povray,wfpokorny/povray,FLOWERCLOUD/povray,davidgiven/povray,c-lipka/povray,LogicalKnight/povray,LogicalKnight/povray,FLOWERCLOUD/povray,davidgiven/povray,UberPOV/UberPOV,UberPOV/UberPOV,FLOWERCLOUD/povray,davidgiven/povray,eblanca/povray,davidgiven/povray,wfpokorny/povray,UberPOV/UberPOV,wfpokorny/povray,FLOWERCLOUD/povray,UberPOV/UberPOV,LogicalKnight/povray,UberPOV/UberPOV,LeForgeron/povray,FLOWERCLOUD/povray,davidgiven/povray,BentSm/povray,eblanca/povray,wfpokorny/povray,LogicalKnight/povray,davidgiven/povray,POV-Ray/povray,eblanca/povray,POV-Ray/povray,POV-Ray/povray,wfpokorny/povray,LeForgeron/povray,LogicalKnight/povray