implement a private state for the uchar.h functions

The C standard is imperative on that:

  7.28.1 ... If ps is a null pointer, each function uses its own internal
  mbstate_t object instead, which is initialized at program startup to
  the initial conversion state;

and these functions are also not supposed to implicitly use the state of
the wchar.h functions:

  7.29.6.3 ... The implementation behaves as if no library function calls
  these functions with a null pointer for ps.

Previously this resulted in two bugs.

 - The functions c16rtomb and mbrtoc16 would crash when called with ps
   set to null.

 - The function mbrtoc32 used the private state of mbrtowc, which it
   is not allowed to do.

src/multibyte/c16rtomb.c

@@ -4,6 +4,8 @@
 
 size_t c16rtomb(char *restrict s, char16_t c16, mbstate_t *restrict ps)
 {
+	static unsigned internal_state;
+	if (!ps) ps = (void *)&internal_state;
 	unsigned *x = (unsigned *)ps;
 	wchar_t wc;
 

src/multibyte/mbrtoc16.c

@@ -3,6 +3,8 @@
 
 size_t mbrtoc16(char16_t *restrict pc16, const char *restrict s, size_t n, mbstate_t *restrict ps)
 {
+	static unsigned internal_state;
+	if (!ps) ps = (void *)&internal_state;
 	unsigned *pending = (unsigned *)ps;
 
 	if (!s) return mbrtoc16(0, "", 1, ps);

src/multibyte/mbrtoc32.c

@@ -3,6 +3,8 @@
 
 size_t mbrtoc32(char32_t *restrict pc32, const char *restrict s, size_t n, mbstate_t *restrict ps)
 {
+	static unsigned internal_state;
+	if (!ps) ps = (void *)&internal_state;
 	if (!s) return mbrtoc32(0, "", 1, ps);
 	wchar_t wc;
 	size_t ret = mbrtowc(&wc, s, n, ps);