srsue: fix stdout metrics print when scells aren't configured yet

during MAC reset, scells are reset to and their PCI is set to UINT32_MAX
which results in malformatted stdout prints, see below:

Random Access Transmission: seq=16, ra-rnti=0x2
Random Access Transmission: seq=14, ra-rnti=0x2
 0   4   -24    24  -1.1u   1.0   140  0.50    0.0     0%   0.0   0.0    0.0    0.0    67%
 14294967295   0.0   0.0  -1.1u   0.0   0.0   0.0    0.0     0%   0.0   0.0    0.0    0.0     0%
 24294967295   0.0   0.0  -1.1u   0.0   0.0   0.0    0.0     0%   0.0   0.0    0.0    0.0     0%
 34294967295   0.0   0.0  -1.1u   0.0   0.0   0.0    0.0     0%   0.0   0.0    0.0    0.0     0%

this patch checks the configured PCI value against UINT32_MAX and
prints "n/a" in case the scells aren't set yet.

srsue/src/metrics_stdout.cc

@@ -115,7 +115,11 @@ void metrics_stdout::set_metrics(const ue_metrics_t& metrics, const uint32_t per
 
   for (uint32_t r = 0; r < metrics.phy.nof_active_cc; r++) {
     cout << std::setw(2) << r;
+    if (metrics.phy.info[r].pci != UINT32_MAX) {
       cout << std::setw(4) << metrics.phy.info[r].pci << std::setw(0);
+    } else {
+      cout << " n/a";
+    }
     cout << float_to_string(metrics.phy.ch[r].rsrp, 2);
     cout << float_to_string(metrics.phy.ch[r].pathloss, 2);
     cout << float_to_eng_string(metrics.phy.sync[r].cfo, 2);

srsue/test/metrics_test.cc

@@ -45,6 +45,7 @@ public:
     m->stack.mac[0].rx_brate  = 200;
     m->stack.mac[0].nof_tti   = 1;
 
+    m->phy.info[1].pci        = UINT32_MAX;
     m->stack.mac[1].rx_pkts   = 100;
     m->stack.mac[1].rx_errors = 100;
     m->stack.mac[1].rx_brate  = 150;