openpgp/elgamal: fix index out of range panic in Decrypt when ciphertext is zero

[nasaa0x]

Decrypt panics when the recovered value s equals zero because
s.Bytes() returns an empty slice and em[0] is indexed without
a bounds check.

An attacker can force s=0 by supplying c2=0 in a crafted
OpenPGP PKESK packet, causing a denial of service.

Fix: check len(em) == 0 before indexing and return an error.

Fixes golang/go#79841
Fixes: https://issuetracker.google.com/issues/519383708

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