encoding: reject negative runes in Encoding.WithPadding
A negative rune (other than NoPadding) makes no semantic sense.
Doing so relies on integer overflow of converting a rune to a byte
and would thus be equivalent to passing the positive byte value
of byte(padding).
This may cause existing code to panic.
An alternative is treat negative runes as equivalent to NoPadding.
However, the code already panics to report erroneous padding values,
so this is in line with the existing API.
Change-Id: I02499705519581598adc0c8525d90e25278dc056
Reviewed-on: https://go-review.googlesource.com/c/go/+/505236
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>