encoding/gob: follow documented io.EOF semantics

The docs say:

	If the input is at EOF, Decode returns io.EOF and does not modify e.

However, the added test fails:

	--- FAIL: TestDecodePartial (0.00s)
		encoder_test.go:1263: 31/81: expected io.ErrUnexpectedEOF: EOF
		encoder_test.go:1263: 51/81: expected io.ErrUnexpectedEOF: EOF

In particular, the decoder would return io.EOF after reading a valid
message for a type specification, and then hit EOF before reading a data
item message.

Fix that by only allowing a Decode call to return io.EOF if the reader
hits EOF immediately, without successfully reading any message.
Otherwise, hitting EOF is an ErrUnexpectedEOF, like in other cases.

Also fix a net/rpc test that, coincidentally, expected an io.EOF
as an error when feeding bad non-zero data to a gob decoder.
An io.ErrUnexpectedEOF is clearly better in that scenario.

Fixes #48905.

Change-Id: Ied6a0d8ac8377f89646319a18c0380c4f2b09b85
Reviewed-on: https://go-review.googlesource.com/c/go/+/354972


Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>