net.c: correct return value in coap_send_internal()

The interface description of coap_send_internal says that COAP_INVALID_MID
is returned  in the case of errors. Presently, it, however, returns
"(coap_mid_t)bytes_written" in a special case. Accidentally,
bytes_written == -1 == COAP_INVALID_MID in this specific case, which is why
no problems occur in practice due to this. As a cosmetic fix, this commit
changes the return value to COAP_INVALID_MID.

src/net.c

@@ -1281,8 +1281,7 @@ coap_send_internal(coap_session_t *session, coap_pdu_t *pdu) {
   }
 
   if (bytes_written < 0) {
-    coap_delete_pdu(pdu);
-    return (coap_mid_t)bytes_written;
+    goto error;
   }
 
 #if !COAP_DISABLE_TCP