vasprintf: stop HEAP exceptions

On Win32, vsnprintf() and vsprintf() are formatting certain things
differently, e.g. "%p". We need to use the same implementation
throughout our vasprintf() replacement.
Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>

include/vlc_fixups.h

@@ -50,7 +50,7 @@ static inline int vasprintf (char **strp, const char *fmt, va_list ap)
     if (res == NULL)
         return -1;
     *strp = res;
-    return vsprintf (res, fmt, ap);
+    return vsnprintf (res, len, fmt, ap);
 #else
     /* HACK: vsnprintf in the WinCE API behaves like
      * the one in glibc 2.0 and doesn't return the number of characters