Windows Meterpreter that uses http/s-based transports wasn't correctly checking for cases where pivoted packets were handled. When pivoted packets are forwarded to the correct handler, the packet is set to NULL. For TCP transports, a check already existed to carry on when the packet was NULL, but this wasn't the case for HTTP/S.
This commit fixes this problem and so the pivot session no longer dies when Meterpreter is using an HTTP/S transport.
For funzies, the fix for this was implemented on a live stream to help other people learn some of Meterp's internals. That video can be found here: https://www.youtube.com/watch?v=de-UYWnafow
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