hex: fix panic in Decode when len(src) > 2*len(dst)

hex.Decode never checks the length of dst and triggers a panic
if there are insufficient bytes in the slice.

There isn't document on what the behavior *should* be in this case.
Two possibilities:
1. Error dst has insufficient space (as done in this change)
2. Reduce the length of the decode to min(dst, src)

Option 1 was chosen because it seems the least surprising or
subtle.

Change-Id: I3bf029e3d928202de716830434285e3c165f26dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/461958
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Benjamin Prosnitz <bprosnitz@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

0 files changed, 0 insertions, 0 deletions