align mbsnrtowcs behavior on partial character with new requirements

POSIX 2024 added a requirement that mbsnrtowcs, like mbrtowc, consume
any final partial character and store it in the mbstate_t object
before returning. this was previously unspecified but documented as a
potential future change.

an internal mbstate_t object is added for the case where the argument
is a null pointer. previously this was not needed since no operations
could modify the internal object and not processing it at all gave the
same behavior "as if" there were an internal object.

src/multibyte/mbsnrtowcs.c

@@ -2,11 +2,13 @@
 
 size_t mbsnrtowcs(wchar_t *restrict wcs, const char **restrict src, size_t n, size_t wn, mbstate_t *restrict st)
 {
+	static unsigned internal_state;
 	size_t l, cnt=0, n2;
 	wchar_t *ws, wbuf[256];
 	const char *s = *src;
 	const char *tmp_s;
 
+	if (!st) st = (void *)&internal_state;
 	if (!wcs) ws = wbuf, wn = sizeof wbuf / sizeof *wbuf;
 	else ws = wcs;
 
@@ -41,8 +43,8 @@ size_t mbsnrtowcs(wchar_t *restrict wcs, const char **restrict src, size_t n, si
 				s = 0;
 				break;
 			}
-			/* have to roll back partial character */
-			*(unsigned *)st = 0;
+			s += n;
+			n -= n;
 			break;
 		}
 		s += l; n -= l;