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)
This is what it'll look like if running the new test:
------DL--------------------------------UL-------------------------------------
rnti cqi ri mcs brate ok nok (%) snr phr mcs brate ok nok (%) bsr
46 16 1.5 28 15.2G 1000 1000 100% 14.2 12.0 20 15.2M 50 10 20% 100.0
ffff 1.2 1.5 6.2 1.00M 100 54 54% 22.2 99.1 28 1.00k 50 10 20% 100.0