Don't fail on libpq-generated error reports in ecpg_raise_backend().

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

src/interfaces/ecpg/ecpglib/error.c

@@ -229,18 +229,17 @@ ecpg_raise_backend(int line, PGresult *result, PGconn *conn, int compat)
 		return;
 	}
 
-	if (result)
-	{
+	/*
+	 * PQresultErrorField will return NULL if "result" is NULL, or if there is
+	 * no such field, which will happen for libpq-generated errors.  Fall back
+	 * to PQerrorMessage in such cases.
+	 */
 	sqlstate = PQresultErrorField(result, PG_DIAG_SQLSTATE);
 	if (sqlstate == NULL)
 		sqlstate = ECPG_SQLSTATE_ECPG_INTERNAL_ERROR;
 	message = PQresultErrorField(result, PG_DIAG_MESSAGE_PRIMARY);
-	}
-	else
-	{
-		sqlstate = ECPG_SQLSTATE_ECPG_INTERNAL_ERROR;
+	if (message == NULL)
 		message = PQerrorMessage(conn);
-	}
 
 	if (strcmp(sqlstate, ECPG_SQLSTATE_ECPG_INTERNAL_ERROR) == 0)
 	{