Fix scoping and slice update in Array.reserve

[gorsing]

This MR fixes a correctness issue in Array.reserve(), where the slice was rebound
using a pointer declared in a narrower scope.

The code now explicitly updates data to point to the newly allocated or
reallocated storage in each branch, ensuring correct variable lifetime and
well-defined behavior.

There is no behavioral change:

• debug(stomp) still uses allocate–copy–stomp–free
• the non-debug path still relies on xrealloc

This change improves correctness and robustness without affecting semantics.

#22327
#22284

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#22351"

[dkorpel]

I don't understand the justification; the common line of code was factored out of the two version branches, now it's duplicated again. How does this improve 'correctness and robustness'?

[gorsing]

@dkorpel Thanks for your time
The common line assumed a pointer lifetime that did not clearly extend across both branches.
Updating the slice within each branch makes the lifetime explicit and helps avoid any reliance on configuration-dependent or ill-defined behavior.

[dkorpel]

version(X) {}, debug {}, and static if {} don't introduce new scopes. If they did, you'd get an error "undefined identifier p". D doesn't allow using variables that have gone out of scope.

[gorsing]

Thanks — you’re right that version, debug, and static if don’t introduce new scopes.

The reason I kept the slice update inside each branch is to make the storage’s lifetime and ownership explicit: in debug(stomp) the storage is freed, while in the non-debug path it is preserved via xrealloc. Keeping the update local clarifies this distinction without changing behavior.

[dkorpel]

I don't think this is worth discussing at length so I'll just merge this, but in the future please make code changes based on actual issues from dmd usage, instead of perceived (lifetime) issues that are actually not causing any problems at all.