Fix SEGV issue in fribidi_remove_bidi_marks
Fix SEGV issue in fribidi_remove_bidi_marks

Escape from fribidi_remove_bidi_marks() immediately if str is null.

This fixes https://github.com/fribidi/fribidi/issues/183

khaledhosny/fribidi,khaledhosny/fribidi,ShiftMediaProject/fribidi,ShiftMediaProject/fribidi,khaledhosny/fribidi,ShiftMediaProject/fribidi