P_LineOpening: set int32 max/min as defaults for opentop, openbottom etc if a...

P_LineOpening: set int32 max/min as defaults for opentop, openbottom etc if a linedef you touched belongs to a polyobjetc. the only thing that really matters in this scenario is the polyobject itself after all!

(This is an untested fix for VAda's apparent collision with thin air below a polyobject in ACZ2 in beta 5)