fix MultiBuffer Compact function

[patterniha]

Unfortunately, the lack of error return in the two functions read and write (in buffer.go) caused the author to write the function Compact (in multi_buffer.go) incorrectly and no one has noticed until now.

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common.Must2(last.ReadFrom(curr)) does not necessarily append curr to last, also common.Must2 has no effect here, because ReadFrom does not return any error on failure.

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for example suppose:

in Compact function for loop, last == mb[0] and curr == mb[1]
and last.start == 2 and last.end == 8192
and curr.start == 0 and curr.end == 2
so last.Len() + curr.Len() <= 8192
but common.Must2(last.ReadFrom(curr)) doesn't actually do anything!!!

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Compact function is used in tls and websocket, and this caused the Xray-core to occasionally send corrupted data for almost all users!!! Whoa!!!

thanks to @Vdu6n1XMrq6694a8 for his/her great Reproduction.

[Fangliding]

buf最好别动 只是 Compact 函数的问题 还是我来修吧