mb_substr() with negative length and offset
mb_substr() with negative length and offset

fixes https://github.com/peachpiecompiler/peachpie/issues/476

iolevel/peachpie-concept,iolevel/peachpie,iolevel/peachpie-concept,iolevel/peachpie-concept,peachpiecompiler/peachpie,iolevel/peachpie,peachpiecompiler/peachpie,peachpiecompiler/peachpie,iolevel/peachpie