get_phy_stats(): full list is non-fatal

Do not assert if the list of phy statistics is full; instead return
NULL.

In most places, this is handled appropriately (we do not store anything
if we can't). In nr_decode_pucch0(), the return value of get_phy_stats()
is used a little bit all over the function, and a constant
  if (uci_stats)
    uci_stats->variable++;
would look ugly, so make uci_stats point to the stack if we cannot store
in the phy_stats.

openair1/PHY/INIT/nr_init.c

@@ -79,8 +79,11 @@ NR_gNB_PHY_STATS_t *get_phy_stats(PHY_VARS_gNB *gNB, uint16_t rnti)
     else if (!stats->active && first_free == -1)
       first_free = i;
   }
+
+  if (first_free < 0)
+    return NULL;
+
   // new stats
-  AssertFatal(first_free >= 0, "PHY statistics list is full\n");
   stats = &gNB->phy_stats[first_free];
   stats->active = true;
   stats->rnti = rnti;