net/http: HTTP/2 Server Crash via HPACK Encoder Race on Table Size Updates

Go’s HTTP/2 server encodes response headers with an HPACK encoder from one goroutine, while another goroutine resizes the same encoder whenever the client sends a SETTINGS_HEADER_TABLE_SIZE value. The two operations were not synchronized. A malicious client that keeps sending requests while repeatedly changing the header table size triggers the data race and crashes the server process, a remote denial of service. The fix applies the new table size at the next frame write instead.