fix aliasing violations in mbtowc and mbrtowc

fix aliasing violations in mbtowc and mbrtowc

these functions were setting wc to point to wchar_t aliasing itself as
a "cheap" way to support null wc arguments. doing so was anything but
cheap, since even without the aliasing violation, it would limit the
compiler's ability to optimize.

making wc point to a dummy object is equally easy and does not suffer
from the above problems.

(cherry picked from commit e89cfe51d2001af08fc2a13e5133ba8157f90beb)