mbsnrtowcs/wcsnrtombs: Fix EILSEQ handling
Originally reported on FreeBSD (PR 209907) by Roel Standaert, RockinRoel
noticed that DragonFly suffered the same bug. When the title functions
encounter a character that cannot be converted, they should change the
src pointer to point to the character positioned immediately after the
failed character, but no such change was performed.
YellowRabbit improved on the FreeBSD patch addressing the bug with a
new version that eliminates possible NULL pointer dereferences.
Dragonfly-bug: <https://bugs.dragonflybsd.org/issues/2920>