Don't use destroyed values in SkArenaAllocWithReset

Found with `MSAN_OPTIONS=poison_in_dtor=1`. Avoid using fields after
`this` has been destroyed.

Change-Id: I3a772ca053c03520268a7e0328402365b5592c40
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/528438
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed