gpg: Cap size of attribute packets at 16MB.

* g10/parse-packet.c (parse_attribute): Avoid xmalloc failure and cap
size of packet.

Tavis Ormandy reported a fatal error for attribute packets with a zero
length payload. This is due to a check in Libgcrypt's xmalloc which
rejects a malloc(0) instead of silently allocating 1 byte. The fix is
obvious.

In addition we cap the size of attribute packets similar to what we do
with user id packets. OpenPGP keys are not the proper way to store
movies.