gpg: Fix ugly error message for an unknown symkey algorithm.

* g10/mainproc.c (proc_symkey_enc): Do not continue with an unknown
algorithm.

Trying to encrypt data created with

printf "\x8c\x49\x05\x0e\x0a\x03\x01"

fails in version 2.2.19 with

gpg: packet(3) with unknown version 5

but with later versions with

gpg: encrypted with unknown algorithm 14
gpg: Ohhhh jeeee: ... this is a bug \
     ([...]/passphrase.c:433:passphrase_to_dek)

so we better catch this case earlier.

• Reported-by: Tavis Ormandy
• Signed-off-by: Werner Koch <wk@gnupg.org>