Fix a return value confusion in chacha20_poly1305_cipher()

Fix a return value confusion in chacha20_poly1305_cipher()

On overlong input, chacha20_poly1305_cipher() would return 0, which in
EVP_CipherUpdate() and EVP_CipherFinal() signals success with no data
written since EVP_CIPH_FLAG_CUSTOM_CIPHER is set. In order to signal an
error, we need to return -1. Obviously.

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