x/net/http2: zero-length payload HEADERS frame causes PROTOCOL_ERROR

[zliu5-sc]

As per RFC7540 Section 6.2:

Padding that exceeds the size remaining for the header block fragment MUST be treated as a PROTOCOL_ERROR.

(emphasis added)

Hence the existing check if len(p)-int(padLength) <= 0 is incorrect
since = would have flagged paddings that is exactly the size
remaining. The existing check also triggers false positives when the
HEADERS frame has a payload size of 0 and no padding (0 - 0 <= 0).

Fixes golang/go#47851

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[zliu5-sc]

close in favor of https://golang.org/cl/344029