mbrtowc.c: Make mbsrtowcs comply with GNU libc when *dst=NULL

mbsrtowcs in GNU libc leaves *src unchanged if *dst=NULL. Currently
the implementation of mbsrtowcs in mingw-w64-crt changes *src just
like when *dst!=NULL. This bug has caused GNU hello to not say
"Hello, world!" when compiled with mingw-w64 and the msys2
distribution.

Signed-off-by: Matthew Palermo <matt.r.palermo@gmail.com>
Signed-off-by: Liu Hao <lh_mouse@126.com>