Fixes jumpUI crash

fixes #420
the second "not" was bugged and not applied to UnitDefs[unitDefID].customParams.canjump == "1" because brackets were missing

if (not groundPos or not(UnitDefs[unitDefID].customParams.canjump == "1")) then
and
if (not groundPos or UnitDefs[unitDefID].customParams.canjump ~= "1") then
would have both worked

Used De Morgan's laws to make it look clearer.
Merge pull request #422 from Skasi/patch-10

Fixes jumpUI crash