http2: fix underflow in http2 server push

[mauri870]

After CL 534215 was merged to fix a CVE it introduced
an underflow when we try to decrement sc.curHandlers
in handlerDone.

The func startPush calls runHandler without incrementing
curHandlers. Seems to only affect users of http.Pusher.

For golang/go#63511

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[gopherbot]

Message from Mauri de Souza Meneguzzo:

Patch Set 2: Run-TryBot+1

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Message from Gopher Robot:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Gopher Robot:

Patch Set 2: TryBot-Result+1

(1 comment)

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[gopherbot]

Message from Dmitri Shuralyov:

Patch Set 2: Code-Review+1

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[gopherbot]

This PR is being closed because golang.org/cl/535595 has been merged.