pgsql: avoid Insert_ID() failing when lastval() is not set

If Insert_ID() is executed before nextval() has been called in the
current session, SELECT lastval() will fail with
`ERROR:  lastval is not yet defined in this session`.

When using Exceptions, this causes one to be thrown but this is not the
expected behavior, as the function is expected to just return false in
this case.

Fixes #978

Signed-off-by: Damien Regad <dregad@mantisbt.org>

Rewritten commit message

drivers/adodb-postgres8.inc.php

@@ -45,11 +45,17 @@ class ADODB_postgres8 extends ADODB_postgres7
 	 * @return int last inserted ID for given table/column, or the most recently
 	 *             returned one if $table or $column are empty
 	 */
-	protected function _insertID($table = '', $column = '')
-	{
-		return empty($table) || empty($column)
-			? $this->GetOne("SELECT lastval()")
-			: $this->GetOne("SELECT currval(pg_get_serial_sequence('$table', '$column'))");
+	protected function _insertID( $table = '', $column = '' ){
+		$sql = 'SELECT ';
+		$sql .= empty($table) || empty($column) ? 'lastval()' : "currval(pg_get_serial_sequence('$table', '$column'))";
+		$result = @$this->GetOne($sql);
+		if( $result === false || $result == $ADODB_GETONE_EOF ){
+			if( $this->debug ){
+				ADOConnection::outp(__FUNCTION__ . "() failed : "  . $this->errorMsg());
+			}
+			return false;
+		}
+		return $result;
 	}
 }