fixes #25992; fix GC tracing of stale bytes in case objects during reset

[ringabout]

fixes #25992

type
  Foo = object
    case kind: bool
    of true:
      a: ref Bar   # 8 bytes (pointer)
    of false:
      b: int       # 4 bytes

specializeResetT for b emits accessor.b = 0 — writes 4 bytes
But the union is 8 bytes wide (sized by the largest branch)
The remaining 4 bytes where a used to live are untouched
Those stale bytes could contain a heap pointer the GC traces → crash

Add nimZeroMem after specializeResetN for case objects to clear the
entire union including unused branch bytes.