pack/symm/symm.go: initAEAD inside symm.Unpack
pack/symm/symm.go: initAEAD inside symm.Unpack

Make sure global var aead is initialized in another code path.

Change-Id: Id7a45f3a6f7f7747b093d6d2871d3f097d079f79
Reviewed-on: https://upspin-review.googlesource.com/5520
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin