lposix.c: simplify gmatch_pushsubject slightly
lposix.c: simplify gmatch_pushsubject slightly

StoneDot/lrexlib,LuaDist/lrexlib-oniguruma,LuaDist/lrexlib-gnu,LuaDist/lrexlib-tre,LuaDist/lrexlib-tre,LuaDist/lrexlib-pcre,LuaDist/lrexlib-posix,LuaDist/lrexlib-posix,LuaDist/lrexlib-pcre,StoneDot/lrexlib,LuaDist/lrexlib-gnu,LuaDist/lrexlib-oniguruma