Prevent Unknown casts in .NET

src/main/java/soot/dotnet/instructions/CilConvInstruction.java

@@ -104,6 +104,11 @@ public Value jimplifyExpr(Body jb) {
         break;
     }
     argument = simplifyComplexExpression(jb, argument);
+    if (convType instanceof UnknownType) {
+      //Decompilation errors might happen. But even in this case, we do not
+      //want to cast to unknown. In this case, let the type assigner figure something out.
+      return argument;
+    }
 
     if (instruction.getCheckForOverflow()) {
       return Jimple.v().newCheckedCastExpr(argument, convType);