lltjfont.sty: append garbage to argument of \jfont only if alternate fonts is set...

lltjfont.sty: append garbage to argument of \jfont only if alternate fonts is set for current Japanese font family

processing time of luatexja-ja.tex
            this commit    previous commit    13/12/30
LuaTeX         10.005            16.955         10.290
LuaJITTeX       8.636            33.199          8.048
(Note that alternate font is not supported in the source at 13/12/30.)

processing time of test05-speed.tex (\count300=30)

         prev commit   13/12/30
LuaTeX      22.593     29.045
LuaJITTeX   16.609     22.982