ssh: allow up to 255 bytes of padding in AES-GCM
ssh: allow up to 255 bytes of padding in AES-GCM

The writing side would generate a maximum of 19 bytes of padding, so
the reading side erroneously checked this. However, RFC 5647 specifies
255 as the maximum amount of padding for AES-GCM.

Fixes golang/go#18953.

Change-Id: I416b0023c6e4cbd91a6a1b4214a03f1663b77248
Reviewed-on: https://go-review.googlesource.com/47590
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
Run-TryBot: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

ScriptRock/crypto,golang/crypto,golang/crypto,golang/crypto,ScriptRock/crypto