fixes stack use after free detected by ASAN
2021-08-31T17:21:44.885938 [MAC-NR ] [D] [ 0] Building new MAC PDU (9 B)
==10908==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7ffc481b5340 at pc 0x563c0486d489 bp 0x7ffc481b4470 sp 0x7ffc481b4460
READ of size 1 at 0x7ffc481b5340 thread T0
#0 0x563c0486d488 in srsran::mac_sch_subpdu_nr::to_string(fmt::v7::basic_memory_buffer<char, 500ul, std::allocator<char> >&) (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x139488)
#1 0x563c0486db87 in srsran::mac_sch_pdu_nr::to_string(fmt::v7::basic_memory_buffer<char, 500ul, std::allocator<char> >&) (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x139b87)
#2 0x563c0481c127 in srsue::mux_nr::get_pdu(unsigned int) (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0xe8127)
#3 0x563c0484e62b in srsue::ul_harq_entity_nr::ul_harq_process_nr::new_grant_ul(srsue::mac_interface_phy_nr::mac_nr_grant_ul_t const&, bool const&, srsue::mac_interface_phy_nr::tb_action_ul_t*) (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x11a62b)
#4 0x563c04850de4 in srsue::ul_harq_entity_nr::new_grant_ul(srsue::mac_interface_phy_nr::mac_nr_grant_ul_t const&, srsue::mac_interface_phy_nr::tb_action_ul_t*) (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x11cde4)
#5 0x563c047bb004 in srsue::mac_nr::new_grant_ul(unsigned int, srsue::mac_interface_phy_nr::mac_nr_grant_ul_t const&, srsue::mac_interface_phy_nr::tb_action_ul_t*) (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x87004)
#6 0x563c04760cdc in msg3_test() (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x2ccdc)
#7 0x563c0475f762 in main (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x2b762)
#8 0x7fae1cf400b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
#9 0x563c047601bd in _start (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x2c1bd)
Address 0x7ffc481b5340 is located in stack of thread T0 at offset 320 in frame
#0 0x563c0486d78f in srsran::mac_sch_pdu_nr::to_string(fmt::v7::basic_memory_buffer<char, 500ul, std::allocator<char> >&) (/home/ubuntu/workspace/srslte_ubuntu_20.04_pull_request/srslte/build/srsue/src/stack/mac_nr/test/mac_nr_test+0x13978f)