test_concurrent_stop_http2 crashes

[rockdaboot]

make check LOG_COMPILER="valgrind --trace-children=yes --leak-check=full --show-leak-kinds=all"

crashes test_concurrent_stop_http2. The log file shows

==15915== Thread 7 MHD-connection:
==15915== Invalid read of size 4
==15915==    at 0x484B61B: build_header_response (connection.c:1681)
==15915==    by 0x48685F1: h2_do_h2_upgrade (h2_upgrade.c:126)
==15915==    by 0x484F0C5: MHD_connection_handle_idle (connection.c:3840)
==15915==    by 0x4851D8E: call_handlers (daemon.c:1140)
==15915==    by 0x4853462: thread_main_handle_connection (daemon.c:2024)
==15915==    by 0x485FD1B: named_thread_starter (mhd_threads.c:275)
==15915==    by 0x4979FA2: start_thread (pthread_create.c:486)
==15915==    by 0x4A8C4CE: clone (clone.S:95)
==15915==  Address 0x90 is not stack'd, malloc'd or (recently) free'd
==15915==
==15915==
==15915== Process terminating with default action of signal 11 (SIGSEGV)
==15915==  Access not within mapped region at address 0x90
==15915==    at 0x484B61B: build_header_response (connection.c:1681)
==15915==    by 0x48685F1: h2_do_h2_upgrade (h2_upgrade.c:126)
==15915==    by 0x484F0C5: MHD_connection_handle_idle (connection.c:3840)
==15915==    by 0x4851D8E: call_handlers (daemon.c:1140)
==15915==    by 0x4853462: thread_main_handle_connection (daemon.c:2024)
==15915==    by 0x485FD1B: named_thread_starter (mhd_threads.c:275)
==15915==    by 0x4979FA2: start_thread (pthread_create.c:486)
==15915==    by 0x4A8C4CE: clone (clone.S:95)
==15915==  If you believe this happened as a result of a stack
==15915==  overflow in your program's main thread (unlikely but
==15915==  possible), you can try to increase the size of the
==15915==  main thread stack using the --main-stacksize= flag.
==15915==  The main thread stack size used in this run was 8388608.