An error PGresult generated by libpq itself, such as a report of
connection loss, won't have broken-down error fields.
ecpg_raise_backend() blithely assumed that PG_DIAG_MESSAGE_PRIMARY
would always be present, and would end up passing a NULL string
pointer to snprintf when it isn't. That would typically crash
before 3779ac62d, and it would fail to provide a useful error report
in any case. Best practice is to substitute PQerrorMessage(conn)
in such cases, so do that.
Per bug #17421 from Masayuki Hirose. Back-patch to all supported
branches.
Discussion: https://postgr.es/m/17421-790ff887e3188874@postgresql.org
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