Coverage issue: ElGamal rarely-hit code path missed in #2151

[ronaldtse]

From @ni4:

Just in case: coverage fails since one of the previous tests hit some ElGamal 'rarely-hit' code path, and now it didn't.

In #2151 this code path seems to be missed causing coverage reduction.

Originally posted by @ni4 in https://github.com/rnpgp/rnp/issues/2151#issuecomment-1845950073