fix #595: If expressions broken in STPL after upgrading

Fixed an error where everything before a block keyword (if/for/...) was
removed by the template parser. Example::

    % m = 'a' if True else 'b'

was translated to::

    if True else 'b'

insead of::

    m = 'a' if True else 'b'
Release of 0.12.5 (bugfix)