compat/vsnprintf: return number of bytes required on truncation.

This conforms to C99, but requires Windows >= XP.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

compat/msvcrt/snprintf.c

@@ -26,6 +26,10 @@
 
 #include "libavutil/error.h"
 
+#if !defined(va_copy) && defined(_MSC_VER)
+#define va_copy(dst, src) ((dst) = (src))
+#endif
+
 int avpriv_snprintf(char *s, size_t n, const char *fmt, ...)
 {
     va_list ap;
@@ -42,9 +46,10 @@ int avpriv_vsnprintf(char *s, size_t n, const char *fmt,
                      va_list ap)
 {
     int ret;
+    va_list ap_copy;
 
     if (n == 0)
-        return 0;
+        return _vscprintf(fmt, ap);
     else if (n > INT_MAX)
         return AVERROR(EOVERFLOW);
 
@@ -55,9 +60,11 @@ int avpriv_vsnprintf(char *s, size_t n, const char *fmt,
      * _snprintf/_vsnprintf() to workaround this problem.
      * See http://msdn.microsoft.com/en-us/library/1kt27hek(v=vs.80).aspx */
     memset(s, 0, n);
-    ret = _vsnprintf(s, n - 1, fmt, ap);
+    va_copy(ap_copy, ap);
+    ret = _vsnprintf(s, n - 1, fmt, ap_copy);
+    va_end(ap_copy);
     if (ret == -1)
-        ret = n;
+        ret = _vscprintf(fmt, ap);
 
     return ret;
 }