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)

src/multibyte/mbrtowc.c

@@ -14,6 +14,7 @@ size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate
 	unsigned c;
 	const unsigned char *s = (const void *)src;
 	const unsigned N = n;
+	wchar_t dummy;
 
 	if (!st) st = (void *)&internal_state;
 	c = *(unsigned *)st;
@@ -21,7 +22,7 @@ size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate
 	if (!s) {
 		if (c) goto ilseq;
 		return 0;
-	} else if (!wc) wc = (void *)&wc;
+	} else if (!wc) wc = &dummy;
 
 	if (!n) return -2;
 	if (!c) {

src/multibyte/mbtowc.c

@@ -12,10 +12,11 @@ int mbtowc(wchar_t *restrict wc, const char *restrict src, size_t n)
 {
 	unsigned c;
 	const unsigned char *s = (const void *)src;
+	wchar_t dummy;
 
 	if (!s) return 0;
 	if (!n) goto ilseq;
-	if (!wc) wc = (void *)&wc;
+	if (!wc) wc = &dummy;
 
 	if (*s < 0x80) return !!(*wc = *s);
 	if (*s-SA > SB-SA) goto ilseq;