buffer: make ssh_buffer_get_len() call ssh_buffer_get_rest_len()

This is a preparatory step for having the behavior of
ssh_buffer_get_rest_len() in the ssh_buffer_get_len() and then remove
the ssh_buffer_rest_len()

Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/buffer.c

@@ -476,7 +476,7 @@ void *ssh_buffer_get(struct ssh_buffer_struct *buffer){
  * @see ssh_buffer_get()
  */
 uint32_t ssh_buffer_get_len(struct ssh_buffer_struct *buffer){
-    return buffer->used;
+    return ssh_buffer_get_rest_len(buffer);
 }
 
 /**