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)
this prepares for a change in which GTPU no longer writes into PDCP
directly but instead uses the stack as a wrapper.
For this the interface will be changed to use the EPS bearer ID instead
of the LCID
The stack will know which PDCP entity (EUTRA or NR) is currently associated
with the EPS bearer ID and will forward the PDU accordingly.