Allows to join two frames into a newly allocated one.
Joins the frame a with b. This is done by concatenating frame a payload followed by frame b payload and adding both payload size, both seqno number. It also checks the more flag for both frames to if it detect some error it will return NULL.
Frame types for both are also checked so if frame type differs a NULL is returned. Channel number for both frames are also checked.
As a result, frame joining done by this function is equal to having activated the complete flag.