Fix an issue with TESTSET on the first line for stripped chunks

src/unluac/decompile/Decompiler.java

@@ -598,13 +598,13 @@ private List<Operation> processLine(State state, int line) {
         break;
       case TEST50: {
         if(getNoDebug() && A != B) {
-          operations.add(new RegisterSet(line, A, Expression.make(Expression.BinaryOperation.OR, r.getExpression(B, line - 1), r.getExpression(A, line - 1))));
+          operations.add(new RegisterSet(line, A, Expression.make(Expression.BinaryOperation.OR, r.getExpression(B, line), initialExpression(state, A, line))));
         }
         break;
       }
       case TESTSET: case TESTSET54: {
         if(getNoDebug()) {
-          operations.add(new RegisterSet(line, A, Expression.make(Expression.BinaryOperation.OR, r.getExpression(B, line - 1), r.getExpression(A, line - 1))));
+          operations.add(new RegisterSet(line, A, Expression.make(Expression.BinaryOperation.OR, r.getExpression(B, line), initialExpression(state, A, line))));
         }
         break;
       }
@@ -751,6 +751,15 @@ private List<Operation> processLine(State state, int line) {
     return operations;
   }
 
+  private Expression initialExpression(State state, int register, int line) {
+    if(line == 1) {
+      if(register < function.numParams) throw new IllegalStateException();
+      return ConstantExpression.createNil(line);
+    } else {
+      return state.r.getExpression(register, line - 1);
+    }
+  }
+
   private Assignment processOperation(State state, Operation operation, int line, int nextLine, Block block) {
     Registers r = state.r;
     boolean[] skip = state.skip;