Fix debug stomping size and safety in Array

[gorsing]

Description: This MR fixes memory stomping logic for debug = stomp builds and improves general safety in memory handling.

• Fix in ~this(): Corrected the memset size argument. It now uses the byte size (length * T.sizeof) instead of the element count (length), fixing incomplete stomping for types where T.sizeof > 1. Added a null check for data.ptr.
• Refactor reserve(): Added checks to skip stomping/clearing unused memory if the array is using inline storage (smallarray) or is uninitialized. This applies to both debug and GC paths.

#22284 #22327

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[dkorpel]

Why do you not want to stomp inline storage?

[gorsing]

@dkorpel Thank you for time.
Good point. I initially added this check to match the logic used for xfree (to be safe), but you are right — we should definitely stomp inline storage as well to catch invalid accesses there. I will remove the smallarray exclusion for the memset call.

I will update it to:

// In reserve method:

debug (stomp)
{
    if (data.ptr) 
    {
        if (length < data.length)
            memset(data.ptr + length, 0xFF, (data.length - length) * T.sizeof);
    }
}

~this() pure nothrow
{
    debug (stomp)
    {
        if (data.ptr)
            memset(data.ptr, 0xFF, data.length * T.sizeof);
    }
    
    if (data.ptr && data.ptr != &smallarray[0])
        mem.xfree(data.ptr);
}