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>

mingw-w64-crt/misc/mbrtowc.c

diff --git a/mingw-w64-crt/misc/mbrtowc.c b/mingw-w64-crt/misc/mbrtowc.c
index c3091cb..2eb4807 100644
--- a/mingw-w64-crt/misc/mbrtowc.c
+++ b/mingw-w64-crt/misc/mbrtowc.c
@@ -137,13 +137,10 @@
   else
     {
       wchar_t byte_bucket = 0;
-      while ((ret = __mbrtowc_cp (&byte_bucket, *src, mb_max,
+      while ((ret = __mbrtowc_cp (&byte_bucket, *src + n, mb_max,
 				     internal_ps, cp, mb_max))
 		  > 0)
-	{
-	  *src += ret;
-	  n += ret;
-	}
+	n += ret;
     }
   return n;
 }