enb: fix potential invalid access in stdout metrics

this fixes a potential access of invalid PHY or MAC metrics by checking
the user entry actually exists.

the RFCI has shown this error:

------DL--------------------------------UL------------------------------------
rnti cqi  ri mcs brate   ok  nok  (%)  snr  phr mcs brate   ok  nok  (%)   bsr
ASAN:DEADLYSIGNAL
=================================================================
m==31838==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x555d482b6893 bp 0x7f6ac32d1160 sp 0x7f6ac32d0bc0 T21)
==31838==The signal is caused by a READ memory access.
==31838==Hint: address points to the zero page.
    #0 0x555d482b6892 in srsenb::metrics_stdout::set_metrics(srsenb::enb_metrics_t const&, unsigned int) /mnt/data/jenkins/workspace/srslte_dev_ogt_zmq_nightly/srsLTE/srsenb/src/metrics_stdout.cc:101
    #1 0x555d482865f1 in srslte::metrics_hub<srsenb::enb_metrics_t>::run_period() /mnt/data/jenkins/workspace/srslte_dev_ogt_zmq_nightly/srsLTE/lib/include/srslte/common/metrics_hub.h:88
    #2 0x555d482865f1 in srslte::periodic_thread::run_thread() /mnt/data/jenkins/workspace/srslte_dev_ogt_zmq_nightly/srsLTE/lib/include/srslte/common/threads.h:143
    #3 0x555d4826813d in srslte:🧵:thread_function_entry(void*) /mnt/data/jenkins/workspace/srslte_dev_ogt_zmq_nightly/srsLTE/lib/include/srslte/common/threads.h:102
    #4 0x7f6b0dc546da in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x76da)
    #5 0x7f6b0bf0171e in __clone (/lib/x86_64-linux-gnu/libc.so.6+0x12171e)

srsenb/src/metrics_stdout.cc

@@ -88,6 +88,12 @@ void metrics_stdout::set_metrics(const enb_metrics_t& metrics, const uint32_t pe
   }
 
   for (size_t i = 0; i < metrics.stack.rrc.ues.size(); i++) {
+    // make sure we have stats for MAC and PHY layer too
+    if (metrics.stack.mac.size() == 0 || metrics.phy.size() == 0 || i > metrics.stack.mac.size() ||
+        i > metrics.phy.size()) {
+      break;
+    }
+
     if (metrics.stack.mac[i].tx_errors > metrics.stack.mac[i].tx_pkts) {
       printf("tx caution errors %d > %d\n", metrics.stack.mac[i].tx_errors, metrics.stack.mac[i].tx_pkts);
     }

srsenb/test/enb_metrics_test.cc

@@ -27,7 +27,7 @@ namespace srsenb {
 
 char* csv_file_name = NULL;
 
-#define NUM_METRICS (3)
+#define NUM_METRICS (4)
 
 // fake classes
 class enb_dummy : public enb_metrics_interface
@@ -100,6 +100,25 @@ public:
     metrics[2].phy[0].dl.mcs  = 28.0;
     metrics[2].phy[0].ul.mcs  = 20.2;
     metrics[2].phy[0].ul.sinr = 14.2;
+
+    // fourth entry with incomple PHY and MAC stats
+    metrics[3].rf.rf_o = 10;
+    metrics[3].stack.rrc.ues.resize(2);
+    metrics[3].stack.mac.resize(metrics[3].stack.rrc.ues.size());
+    metrics[3].stack.mac[0].rnti      = 0x1;
+    metrics[3].stack.mac[0].tx_pkts   = 9999;
+    metrics[3].stack.mac[0].tx_errors = 1;
+    metrics[3].stack.mac[0].tx_brate  = 776;
+    metrics[3].stack.mac[0].rx_pkts   = 50;
+    metrics[3].stack.mac[0].rx_errors = 0;
+    metrics[3].stack.mac[0].rx_brate  = 1e3;
+    metrics[3].stack.mac[0].ul_buffer = 100;
+    metrics[3].stack.mac[0].dl_buffer = 200;
+    metrics[3].stack.mac[0].dl_cqi    = 15.9;
+    metrics[3].stack.mac[0].dl_ri     = 1.5;
+    metrics[3].stack.mac[0].dl_pmi    = 1.0;
+    metrics[3].stack.mac[0].phr       = 12.0;
+    metrics[3].phy.resize(0); // no PHY metrics for this UE
   }
 
   bool get_metrics(enb_metrics_t* m)