crt/mingw_pformat: Initialize `mbstate_t` structure properly

When `wcrtomb()` is called with a null wide character (second argument) it is
required to output a sequence of bytes to 'restore the initial shift state'.
For this function, the `mbstate_t` structure shall be initialized before the
call.

Previous code passed an uninitialized structure and expected `wcrtomb(buf,
L'\0', &state)` to initialize `state` to the initial shift state, which was
incorrect.

Signed-off-by: LIU Hao <lh_mouse@126.com>

mingw-w64-crt/stdio/mingw_pformat.c

diff --git a/mingw-w64-crt/stdio/mingw_pformat.c b/mingw-w64-crt/stdio/mingw_pformat.c
index d094e7a..4e41b4d 100644
--- a/mingw-w64-crt/stdio/mingw_pformat.c
+++ b/mingw-w64-crt/stdio/mingw_pformat.c
@@ -569,8 +569,8 @@
    * output quota is honoured.
    */
   char buf[16];
-  mbstate_t state;
-  int len = wcrtomb(buf, L'\0', &state);
+  mbstate_t state = {0};
+  int len;
 
   if( (stream->precision >= 0) && (count > stream->precision) )
     /*