Fix potential UAF in StunDictionaryWriter::CreateDelta + heap exhaustion in StunDictionaryView::ApplyDelta

1) The UAF is possible if Write used Set + Delete in same Delta,
when later cleaning the delta (in ApplyDeltaAck) the state was
left inconsistently so that subsequent CreateDelta could experience
UAF. It is theoretically possible that an actual WebRTC program could do
this, but I'm quite certain we currently *dont* use Delete.

2) The heap exhaustion in StunDictionaryView::ApplyDelta after
crafting a malicious delta. This is not possible to do with existing
code, but obviously an attacker could it learning about the ice
ufra/passwd.

BUG=b/513584807,b/513584726

Change-Id: Ic286ad4a7cc6c127fdcd3359505898a36eabdfee
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474600
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47777}