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.