Fix in-place decryption for EVP_chacha20_poly1305()

Fix in-place decryption for EVP_chacha20_poly1305()

Take the MAC before clobbering the input value on decryption. Fixes hangs
during the QUIC handshake with HAProxy using TLS_CHACHA20_POLY1305_SHA256.

Found, issue pinpointed, and initial fix tested by Lucas Gabriel Vuotto:
Let me take this opportunity to thank the HAProxy team for going out of
their way to keep supporting LibreSSL. It's much appreciated.

See https://github.com/haproxy/haproxy/issues/2569

tweak/ok jsing