bugfix: aesgcm_used --> enc_aesgcm_used

[ericcgu]

Fix Variable Typo #3085

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[drwetter]

@dcooper16 : ok?

[dcooper16]

@dcooper16 : ok?

It seems ok to me.

The code being fixed in this PR was added by #2796. I don't understand the reason that $enc_chacha_used and $enc_aesgcm_used were defined. As the code is written (once corrected by this PR), $enc_chacha_used or $enc_aesgcm_used will always be true if $plaintext is not empty. So, it seems unnecessary to set one of these variables, and then check its value in addition to checking if $plaintext is not empty.