Fix not to SEGV ibus-ui-gtk3 with wrong 'preload-engines' value.

Fedora internal patch could save engines from the result of
'setxkbmap -query' but they do not exist in simple.xml likes
'xkb:cn::chi' while now the current implementation converts
those engines to 'xkb:us::eng'.

BUG=https://code.google.com/p/ibus/issues/detail?id=1744
TEST=ui/gtk3

Review URL: https://codereview.appspot.com/158640043