mac_pdu: fix check of remaining length when adding new subheader to MAC PDU

when adding a new subheader to a MAC PDU we should only add the subheader
if at least 2 bytes are left in the PDU. Even a 1 Byte MAC Control Element requires
another byte to pack the subheader.

This fixes #1424 in which a 3 B MAC opportunity is provided in the UL. After
adding the Short BSR, we tried to add another MAC subheader. Adding the subheader
succeeded even though no space is left to add a single byte. With this patch,
adding the subheader fails and the generates MAC PDU is correct.

lib/include/srslte/mac/pdu.h

@@ -165,7 +165,9 @@ public:
 
   bool new_subh()
   {
-    if (nof_subheaders < (int)max_subheaders - 1 && rem_len > 0 && buffer_tx->get_headroom() > 1) {
+    if (nof_subheaders < (int)max_subheaders - 1 &&
+        rem_len >= 2 /* Even a MAC CE with one byte (e.g. PHR) needs a 1 Byte subheader */
+        && buffer_tx->get_headroom() > 1) {
       nof_subheaders++;
       return next();
     } else {