Fix contness of debug_print_buf()

library/debug.c

@@ -120,7 +120,7 @@ void debug_print_ret( const ssl_context *ssl, int level,
 
 void debug_print_buf( const ssl_context *ssl, int level,
                       const char *file, int line, const char *text,
-                      unsigned char *buf, size_t len )
+                      const unsigned char *buf, size_t len )
 {
     char str[512];
     char txt[17];