[Listener] Add an assert, improve 2 comments

owlps-listener/owlps-listener.h

@@ -162,6 +162,8 @@ enum {MODE_ACTIVE = 'a', MODE_PASSIVE = 'p', MODE_MIXED = 'm'} ;
  * Returns the number of bytes needed to jump after a (radiotap) field of
  * length 'FIELD_LEN' that should sit at the position 'OFFSET' in the
  * buffer, taking into account alignment considerations.
+ * Warning: do *not* pass an incremented number as FIELD_LEN (e.g.
+ * field_len++ or ++field_len). This shouldn't normally be useful anyway.
  */
 #define SKIP_FIELD(OFFSET, FIELD_LEN) \
   ((FIELD_LEN) + nat_align((OFFSET), (FIELD_LEN)))

owlps-listener/owlps-listenerd.c

@@ -1698,7 +1698,8 @@ void send_autocalibration_request()
 
 /*
  * Creates the calibration packet to send.
- * The packet must be freed by the calling function.
+ * 'packet' must not be NULL. '*packet' must be freed by the calling
+ * function.
  * Returns the size of the packet.
  * In case of error, 0 is returned and *packet is not updated.
  */
@@ -1714,6 +1715,8 @@ uint_fast16_t make_packet(uint8_t **packet)
     my_position_z = owl_htonf(GET_MY_POSITION_Z()) ;
   uint16_t npkt ;
 
+  assert(packet) ;
+
   owl_timestamp_now(&request_time) ;
 
   if (VERBOSE_CHATTERBOX)