pgcrypto: Check for error return of px_cipher_decrypt()

pgcrypto: Check for error return of px_cipher_decrypt()

This has previously not been a problem (that anyone ever reported),
but in future OpenSSL versions (3.0.0), where legacy ciphers are/can
be disabled, this is the place where this is reported.  So we need to
catch the error here, otherwise the higher-level functions would
return garbage.  The nearby encryption code already handled errors
similarly.

Author: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/9e9c431c-0adc-7a6d-9b1a-915de1ba3fe7@enterprisedb.com
Backpatch-through: 9.6