fix: RangeError flood when the MSE staging buffer overflows

[paolostivanin]

video-rtc.js buffers incoming websocket frames into a fixed 2 MiB Uint8Array while the SourceBuffer is busy, and drains it on updateend. Two gaps make that wedge permanently:

• if the drain's appendBuffer throws (QuotaExceededError is the common one, when a mobile browser backgrounds the tab and the decoder stops evicting), the exception is swallowed and bufLen is never reset. No update cycle was started, so updateend never fires again and the drain is never re-entered.
• ondata then always takes the buffering branch, and buf.set(b, bufLen) has no bounds check. Once bufLen passes 2 MiB it throws "RangeError: offset is out of bounds" on every frame that arrives, at stream frame rate, until the socket closes.

Observed as ~150 errors/second reported back to the server by Home Assistant's frontend error reporter.

Reset bufLen when the drain fails, and bound-check the staging append, dropping the backlog and restarting the update cycle so the stream can resync on the next keyframe.