Functions XORKeyStream should panic if len(dst) < len(src), but it
write to dst before bounds checking. In asm routines and fastXORBytes,
this is an out of bounds write.

Fixes #21104

Change-Id: I354346cda8d63910f3bb619416ffd54cd0a04a0b
Reviewed-on: https://go-review.googlesource.com/52050Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>