Fix typo

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1378,7 +1378,7 @@ object Types {
     def followOpaqueGADT(implicit ctx: Context): Type = widenDealias match {
       case site: TypeRef if site.symbol.is(Opaque) =>
         val bounds = ctx.gadt.bounds(site.symbol)
-        if (bounds != 0) bounds.hi else NoType
+        if (bounds != null) bounds.hi else NoType
       case AppliedType(tycon, args) =>
         val tycon1 = tycon.followOpaqueGADT
         if (tycon1.exists) tycon1.appliedTo(args)