rlc am - fix bugs in buffered_pdcp_pdu_list

The count and container elements were not being correctly updated in the events
of SN overwrite, clear(), clear_pdcp_sdu(sn) of sn that doesnt exist

lib/include/srsran/upper/rlc_am_lte.h

@@ -116,13 +116,20 @@ public:
 
   void add_pdcp_sdu(uint32_t sn)
   {
-    assert(not has_pdcp_sn(sn));
+    srsran_assert(not has_pdcp_sn(sn), "Cannot re-add same PDCP SN twice");
+    uint32_t sn_idx = get_idx(sn);
+    if (buffered_pdus[sn_idx].sn != invalid_sn) {
+      clear_pdcp_sdu(buffered_pdus[sn_idx].sn);
+    }
     buffered_pdus[get_idx(sn)].sn = sn;
     count++;
   }
   void clear_pdcp_sdu(uint32_t sn)
   {
     uint32_t sn_idx = get_idx(sn);
+    if (buffered_pdus[sn_idx].sn == invalid_sn) {
+      return;
+    }
     buffered_pdus[sn_idx].sn          = invalid_sn;
     buffered_pdus[sn_idx].fully_acked = false;
     buffered_pdus[sn_idx].fully_txed  = false;

lib/src/upper/rlc_am_lte.cc

@@ -2128,6 +2128,7 @@ buffered_pdcp_pdu_list::buffered_pdcp_pdu_list() : buffered_pdus(max_buffer_idx
 
 void buffered_pdcp_pdu_list::clear()
 {
+  count = 0;
   for (auto& b : buffered_pdus) {
     b.sn          = invalid_sn;
     b.fully_acked = false;