Return a negative value to indicate truncation.

If the maximum availalbe buffer size is zero, no data can be written
(not even the NULL terminator), and a negative values should be returned
to indicate truncation.

src/context.c

@@ -50,7 +50,7 @@ l_vsnprintf (char *str, size_t size, const char *format, va_list ap)
 	int n;
 
 	if (size == 0)
-		return 0;
+		return -1;
 
 #ifdef _MSC_VER
 	/*