Fix memory leak introduced in March

This memory leak was introduced in
205dc65.

This results in exactly the same number of ASAN errors (16) as completely
reverting that commit.

src/eval.cpp

@@ -960,8 +960,8 @@ namespace Sass {
  }
 
  if (Cast<String_Schema>(c->sname())) {
- Expression_Ptr evaluated_name = c->sname()->perform(this);
- Expression_Ptr evaluated_args = c->arguments()->perform(this);
+ Expression_Obj evaluated_name = c->sname()->perform(this);
+ Expression_Obj evaluated_args = c->arguments()->perform(this);
  std::string str(evaluated_name->to_string());
  str += evaluated_args->to_string();
  return SASS_MEMORY_NEW(String_Constant, c->pstate(), str);