This is to keep the local variables of the previously created blocks consistent in case the `mrbc_context` passed to `mrb_load_exec()` is `NULL` or different.
Switching between `mrbc_context` pointers that are non `NULL` can be done safely by calling `mrbc_cleanup_local_variables()`.

Before this patch, the result of the following code is not as expected.

```console
% cat loadstr.c
#include <mruby.h>
#include <mruby/compile.h>

int
main(int argc, char *argv[])
{
  mrb_state *mrb = mrb_open();

  mrb_load_string(
      mrb,
      "(a, b, c, d, e, f, g) = [1, 2, 3, 4, 5, 6, 7] \n"
      "$lambda = -> { p [a, b, c, d, e, f, g] }");
  mrb_load_string(mrb, "$lambda.call");

  mrb_close(mrb);

  return 0;
}

% $(bin/mruby-config --cc --cflags --ldflags) loadstr.c $(bin/mruby-config --libs) && ./a.out
[main, nil, nil, main, nil, nil, main]
```

Also, since `mrb_env_unshare()` was not used before, the internal stack of simply detached `env` objects could show invalid addresses by `stack_extend()`.
ref. https://github.com/kou/mruby-pp/commit/ef5951aca870183d8767cb61f6414240988ca35e