fix: prevent panic on closed channel onPing

[avonbirgelen]

Proposed changes

I am experiencing a panic in some situations related to the Websocket Ping/Pong.
It seems that in some cases (i assume unstable connections but I have been unable to reproduce this in my tests), the connection is being cleaned up and the OnPing still tries to write to the pingC channel, resulting in "panic: send on closed channel".

This should fix that issue.

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[avonbirgelen]

On Second thought. Not sure if this is enough. I have not been able to reliably reproduce the error. So I am unsure if this catches the panic, or just reacts when the channel is full.
Will do some more testing on my end and update this PR. Or feel free to close it.