Reference to CollectSplat @parts should be @array

It looks like this reference to `@parts` is leftover from a refactoring.  Judging from context, it is probably meant to reference `@array`.

Obviously, this is a problem for sexp generation, because `@parts` is `nil`, and cannot be `map`ped.

```
> rbx compile -S -e "x(*y, z)"
An exception occurred running compile:

    undefined method `map' on nil:NilClass. (NoMethodError)

Backtrace:

  Kernel(NilClass)#map (method_missing) at kernel/delta/kernel.rb:78
  CodeTools::AST::CollectSplat#to_sexp at /home/jemc/.rbenv/versions/rbx-master
                                          /runtime/gems/rubinius-ast-2.2.5/lib
                                          /rubinius/ast/sends.rbc:426
     CodeTools::AST::Arguments#to_sexp at /home/jemc/.rbenv/versions/rbx-master
                                          /runtime/gems/rubinius-ast-2.2.5/lib
                                          /rubinius/ast/sends.rbc:505
 CodeTools::AST::SendWithArguments#arguments_sexp at /home/jemc/.rbenv/versions
                                                     /rbx-master/runtime/gems
                                                     /rubinius-ast-2.2.5/lib
                                                     /rubinius/ast/sends.rbc:161
  CodeTools::AST::Send(CodeTools::AST::SendWithArguments)#to_sexp at \
          /home/jemc/.rbenv/versions/rbx-master/runtime/gems/rubinius-ast-2.2.5/lib/rubinius/ast/sends.rbc:123
 CodeTools::AST::Container(CodeTools::AST::Script)#to_sexp at /home/jemc/.rbenv
                                                              /versions/rbx-master
                                                              /runtime/gems
                                                              /rubinius-ast-2.2.5
                                                              /lib/rubinius/ast
                                                              /definitions.rbc:1154
  CodeTools::Compiler::SexpPrinter#run at /home/jemc/.rbenv/versions/rbx-master
                                          /runtime/gems/rubinius-compiler-2.2.1/lib
                                          /rubinius/compiler/printers.rbc:22
  CodeTools::Compiler::Stage(CodeTools::Compiler::StringParser)#run_next at \
          /home/jemc/.rbenv/versions/rbx-master/runtime/gems/rubinius-compiler-2.2.1/lib/rubinius/compiler/stages.rbc:53
  CodeTools::Compiler::Parser(CodeTools::Compiler::StringParser)#run at \
          /home/jemc/.rbenv/versions/rbx-master/runtime/gems/rubinius-compiler-2.2.1/lib/rubinius/compiler/stages.rbc:210
               CodeTools::Compiler#run at /home/jemc/.rbenv/versions/rbx-master
                                          /runtime/gems/rubinius-compiler-2.2.1/lib
                                          /rubinius/compiler/compiler.rbc:361
         CompilerScript#compile_string at /home/jemc/.rbenv/versions/rbx-master
                                          /library/bin/compile.rb:219
 { } in CompilerScript#compile_strings at /home/jemc/.rbenv/versions/rbx-master
                                          /library/bin/compile.rb:224
                             Array#each at kernel/bootstrap/array.rb:76
        CompilerScript#compile_strings at /home/jemc/.rbenv/versions/rbx-master
                                          /library/bin/compile.rb:223
                   CompilerScript#main at /home/jemc/.rbenv/versions/rbx-master
                                          /library/bin/compile.rb:235
                     Object#__script__ at /home/jemc/.rbenv/versions/rbx-master
                                          /library/bin/compile.rb:240
       Rubinius::CodeLoader#load_script at kernel/delta/code_loader.rb:66
       Rubinius::CodeLoader.load_script at kernel/delta/code_loader.rb:152
                Rubinius::Loader#script at kernel/loader.rb:649
                  Rubinius::Loader#main at kernel/loader.rb:827
```