internal/saferio: avoid returning io.EOF from ReadData if data was read

internal/saferio: avoid returning io.EOF from ReadData if data was read

ReadData follows the error semantics of io.ReadFull for small sizes,
it should do so as well for large sizes.

Change-Id: I6a11b00d903ac5332e1dda074473790dcf21f32a
Reviewed-on: https://go-review.googlesource.com/c/go/+/424919
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>