Incorrect streaming in sysprof when fast function is on top

How to reproduce (isn't stable).

Apply the following patch:

diff --git a/test/config-luatest/CMakeLists.txt b/test/config-luatest/CMakeLists.txt
index 6c13cb810..8fac27489 100644
--- a/test/config-luatest/CMakeLists.txt
+++ b/test/config-luatest/CMakeLists.txt
@@ -29,6 +29,7 @@ foreach(test_path ${tests})
                    # Disable stdout buffering,
                    # see https://github.com/tarantool/test-run/issues/119.
                    -e "io.stdout:setvbuf('no')"
+                   -e "misc.sysprof.start({mode = 'C', path = '/dev/null', interval = 1})"
                    ${LUATEST_COMMAND_WITH_FLAGS}
                    --shuffle all:${RANDOM_SEED}
                    ${test_title}

Build Tarantool like the following:

cmake -DCMAKE_BUILD_TYPE=Debug -DLUA_USE_APICHECK=ON -DLUA_USE_ASSERT=ON -DLUAJIT_ENABLE_GC64=OFF -DENABLE_ASAN=OFF . && make tarantool -j

And run

ctest -R test/config-luatest/anonymous_replica_test.lua -V

Backtrace:

(gdb) bt
#0  0x00007f9847ca4f0c in __pthread_kill_implementation () from /lib64/libc.so.6
#1  0x00007f9847c4fb36 in raise () from /lib64/libc.so.6
#2  0x00007f9847c378fa in abort () from /lib64/libc.so.6
#3  0x00005645c2dab9db in crash_signal_cb (signo=11, siginfo=0x5645eccd53f0, context=0x5645eccd52c0)
    at src/lib/core/crash.c:204
#4  <signal handler called>
#5  0x00005645c2e84b48 in stream_lfunc (buf=0x5645c36330d8 <sysprof+24>, func=0x4188f9e0)
    at third_party/luajit/src/lj_sysprof.c:128
#6  0x00005645c2e84cc6 in stream_frame_lua (buf=0x5645c36330d8 <sysprof+24>, frame=0x40bcf068)
    at third_party/luajit/src/lj_sysprof.c:149
#7  0x00005645c2e84eee in stream_backtrace_lua (sp=0x5645c36330c0 <sysprof>, vmstate=2)
    at third_party/luajit/src/lj_sysprof.c:188
#8  0x00005645c2e852fc in stream_guest (sp=0x5645c36330c0 <sysprof>, vmstate=2)
    at third_party/luajit/src/lj_sysprof.c:256
#9  0x00005645c2e85405 in stream_event (sp=0x5645c36330c0 <sysprof>, vmstate=2)
    at third_party/luajit/src/lj_sysprof.c:291
#10 0x00005645c2e8550f in sysprof_record_sample (sp=0x5645c36330c0 <sysprof>, info=0x7f9845680930)
    at third_party/luajit/src/lj_sysprof.c:321
#11 0x00005645c2e855c5 in sysprof_signal_handler (sig=27, info=0x7f9845680930, ctx=0x7f9845680800)
    at third_party/luajit/src/lj_sysprof.c:337
#12 <signal handler called>
#13 0x00005645c2e1bfcb in lj_fff_res () at buildvm_x86.dasc:1792
#14 0x00005645c2e28a2b in lua_pcall (L=0x40bcb4d8, nargs=3, nresults=-1, errfunc=0)
    at third_party/luajit/src/lj_api.c:1173
#15 0x00005645c2d862ed in luaT_call (L=0x40bcb4d8, nargs=3, nreturns=-1) at src/lua/utils.c:708
#16 0x00005645c2d7b08e in lua_fiber_run_f (ap=0x7f9847012228) at src/lua/fiber.c:452
#17 0x00005645c2ac617c in fiber_cxx_invoke(fiber_func, typedef __va_list_tag __va_list_tag *) (f=0x5645c2d7afd1 <lua_fiber_run_f>, ap=0x7f9847012228)
    at src/lib/core/fiber.h:1324
#18 0x00005645c2db7863 in fiber_loop (data=0x0) at src/lib/core/fiber.c:1167
#19 0x00005645c31e9cd3 in coro_init () at third_party/coro/coro.c:108

The top_frame value is the following. Hence, the invalid stack traversing takes place.

(gdb) lj-tv top_frame
fast function #4 ; next

Start with third_party/luajit/src/lj_sysprof.c, especially stream_lfunc(), stream_frame_lua(), and stream_backtrace_lua(), then reproduce with the patch to test/config-luatest/CMakeLists.txt and the anonymous_replica_test.lua command. Done means the sysprof run no longer performs invalid stack traversal or crashes when a fast function is on top.